lib/fs: Optimize WindowsInvalidFilename (#8687)

Replaced strings.Split with the new strings.Cut, which doesn't allocate.

name                           old time/op    new time/op    delta
WindowsInvalidFilenameValid-8     154ns ± 2%      89ns ± 0%   -42.09%  (p=0.000 n=10+9)
WindowsInvalidFilenameNUL-8       124ns ± 2%     124ns ± 1%      ~     (p=0.371 n=8+10)

name                           old alloc/op   new alloc/op   delta
WindowsInvalidFilenameValid-8     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)
WindowsInvalidFilenameNUL-8       19.0B ± 0%      3.0B ± 0%   -84.21%  (p=0.000 n=10+10)

name                           old allocs/op  new allocs/op  delta
WindowsInvalidFilenameValid-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
WindowsInvalidFilenameNUL-8        2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=10+10)

lib/fs/util.go

@@ -53,9 +53,17 @@ const windowsDisallowedCharacters = (`<>:"|?*` +
 	"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f")
 
 func WindowsInvalidFilename(name string) error {
+	// The path must not contain any disallowed characters.
+	if strings.ContainsAny(name, windowsDisallowedCharacters) {
+		return errInvalidFilenameWindowsReservedChar
+	}
+
 	// None of the path components should end in space or period, or be a
 	// reserved name.
-	for _, part := range strings.Split(name, `\`) {
+	for len(name) > 0 {
+		part, rest, _ := strings.Cut(name, `\`)
+		name = rest
+
 		if part == "" {
 			continue
 		}
@@ -69,11 +77,6 @@ func WindowsInvalidFilename(name string) error {
 		}
 	}
 
-	// The path must not contain any disallowed characters
-	if strings.ContainsAny(name, windowsDisallowedCharacters) {
-		return errInvalidFilenameWindowsReservedChar
-	}
-
 	return nil
 }