Prevent oopsing in posix_acl_valid()

If posix_acl_from_xattr() returns an error code, a negative address is
dereferenced causing an oops; fix by checking for error code first.
Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>
Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/acl.c

@@ -178,12 +178,13 @@ static int btrfs_xattr_acl_set(struct dentry *dentry, const char *name,
 
 	if (value) {
 		acl = posix_acl_from_xattr(value, size);
+		if (IS_ERR(acl))
+			return PTR_ERR(acl);
+
 		if (acl) {
 			ret = posix_acl_valid(acl);
 			if (ret)
 				goto out;
-		} else if (IS_ERR(acl)) {
-			return PTR_ERR(acl);
 		}
 	}