Btrfs: deal with NULL acl sent to btrfs_set_acl

It is legal for btrfs_set_acl to be sent a NULL acl.  This
makes sure we don't dereference it.  A similar patch was sent by
Johannes Hirte <johannes.hirte@fem.tu-ilmenau.de>

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/acl.c

@@ -112,12 +112,14 @@ static int btrfs_set_acl(struct btrfs_trans_handle *trans,
 	switch (type) {
 	case ACL_TYPE_ACCESS:
 		mode = inode->i_mode;
-		ret = posix_acl_equiv_mode(acl, &mode);
-		if (ret < 0)
-			return ret;
-		ret = 0;
-		inode->i_mode = mode;
 		name = POSIX_ACL_XATTR_ACCESS;
+		if (acl) {
+			ret = posix_acl_equiv_mode(acl, &mode);
+			if (ret < 0)
+				return ret;
+			inode->i_mode = mode;
+		}
+		ret = 0;
 		break;
 	case ACL_TYPE_DEFAULT:
 		if (!S_ISDIR(inode->i_mode))