arm64: idreg-override: avoid strlen() to check for empty strings

strlen() is a costly way to decide whether a string is empty, as in that
case, the first character will be NUL so we can check for that directly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-60-ardb@google.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: ardbiesheuvel

arch/arm64/kernel/idreg-override.c

@@ -232,7 +232,7 @@ static void __init match_options(const char *cmdline)
 
 		override = prel64_pointer(reg->override);
 
-		for (f = 0; strlen(reg->fields[f].name); f++) {
+		for (f = 0; reg->fields[f].name[0] != '\0'; f++) {
 			u64 shift = reg->fields[f].shift;
 			u64 width = reg->fields[f].width ?: 4;
 			u64 mask = GENMASK_ULL(shift + width - 1, shift);