kdb: Replace strncmp with str_has_prefix

strncmp(str, const, len) is error-prone.
We had better use newly introduced
str_has_prefix() instead of it.
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

kernel/debug/kdb/kdb_main.c

@@ -830,7 +830,7 @@ static void parse_grep(const char *str)
 	cp++;
 	while (isspace(*cp))
 		cp++;
-	if (strncmp(cp, "grep ", 5)) {
+	if (!str_has_prefix(cp, "grep ")) {
 		kdb_printf("invalid 'pipe', see grephelp\n");
 		return;
 	}