9p v9fs_parse_options: replace simple_strtoul with kstrtouint

This is also a checkpatch change, but this one might have more implications
so keeping this separate

Link: http://lkml.kernel.org/r/20211102134608.1588018-4-dominique.martinet@atmark-techno.comSigned-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/v9fs.c

@@ -164,7 +164,7 @@ static int v9fs_parse_options(struct v9fs_session_info *v9ses, char *opts)
 	substring_t args[MAX_OPT_ARGS];
 	char *p;
 	int option = 0;
-	char *s, *e;
+	char *s;
 	int ret = 0;
 
 	/* setup defaults */
@@ -319,12 +319,13 @@ static int v9fs_parse_options(struct v9fs_session_info *v9ses, char *opts)
 				v9ses->flags |= V9FS_ACCESS_CLIENT;
 			} else {
 				uid_t uid;
+
 				v9ses->flags |= V9FS_ACCESS_SINGLE;
-				uid = simple_strtoul(s, &e, 10);
-				if (*e != '\0') {
-					ret = -EINVAL;
-					pr_info("Unknown access argument %s\n",
-						s);
+				r = kstrtouint(s, 10, &uid);
+				if (r) {
+					ret = r;
+					pr_info("Unknown access argument %s: %d\n",
+						s, r);
 					kfree(s);
 					continue;
 				}