kernel: Fix boot of amrsr target in qemu

The bootup of the armsr target sometimes failed like this:
```
[    0.762283] NET: Registered PF_INET6 protocol family
[    0.791987] workqueue: Failed to create a worker thread: -EAGAIN
[    0.801986] workqueue: Failed to create a rescuer kthread for wq "mld": -EAGAIN
[    0.964017] NET: Unregistered PF_INET6 protocol family
```

Increase the maximum number of threads the kernel allows.

Link: https://github.com/openwrt/openwrt/pull/19376
Signed-off-by: Hauke Mehrtens <[email protected]>

target/linux/generic/pending-6.12/250-kernel-fork-Increase-minimum-number-of-allowed-threa.patch

@@ -0,0 +1,37 @@
+From a22c4c7bbfad3e93e717e16f6faf343259add27a Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <[email protected]>
+Date: Fri, 27 Jun 2025 11:09:04 +0100
+Subject: [PATCH v2] kernel/fork: Increase minimum number of allowed threads
+
+A modern Linux system creates much more than 20 threads at bootup.
+When I booted up OpenWrt in qemu the system sometimes failed to boot up
+when it wanted to create the 419th thread. The VM had 128MB RAM and the
+calculation in set_max_threads() calculated that max_threads should be
+set to 419. When the system booted up it tried to notify the user space
+about every device it created because CONFIG_UEVENT_HELPER was set and
+used. I counted 1299 calls to call_usermodehelper_setup(), all of
+them try to create a new thread and call the userspace hotplug script in
+it.
+
+This fixes bootup of Linux on systems with low memory.
+
+I saw the problem with qemu 10.0.2 using these commands:
+qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic
+
+Cc: [email protected]
+Signed-off-by: Hauke Mehrtens <[email protected]>
+---
+ kernel/fork.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/kernel/fork.c
++++ b/kernel/fork.c
+@@ -123,7 +123,7 @@
+ /*
+  * Minimum number of threads to boot the kernel
+  */
+-#define MIN_THREADS 20
++#define MIN_THREADS 600
+ 
+ /*
+  * Maximum number of threads

target/linux/generic/pending-6.6/250-kernel-fork-Increase-minimum-number-of-allowed-threa.patch

@@ -0,0 +1,37 @@
+From a22c4c7bbfad3e93e717e16f6faf343259add27a Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <[email protected]>
+Date: Fri, 27 Jun 2025 11:09:04 +0100
+Subject: [PATCH v2] kernel/fork: Increase minimum number of allowed threads
+
+A modern Linux system creates much more than 20 threads at bootup.
+When I booted up OpenWrt in qemu the system sometimes failed to boot up
+when it wanted to create the 419th thread. The VM had 128MB RAM and the
+calculation in set_max_threads() calculated that max_threads should be
+set to 419. When the system booted up it tried to notify the user space
+about every device it created because CONFIG_UEVENT_HELPER was set and
+used. I counted 1299 calls to call_usermodehelper_setup(), all of
+them try to create a new thread and call the userspace hotplug script in
+it.
+
+This fixes bootup of Linux on systems with low memory.
+
+I saw the problem with qemu 10.0.2 using these commands:
+qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic
+
+Cc: [email protected]
+Signed-off-by: Hauke Mehrtens <[email protected]>
+---
+ kernel/fork.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/kernel/fork.c
++++ b/kernel/fork.c
+@@ -115,7 +115,7 @@
+ /*
+  * Minimum number of threads to boot the kernel
+  */
+-#define MIN_THREADS 20
++#define MIN_THREADS 600
+ 
+ /*
+  * Maximum number of threads