btrfs: fix missing error return in btrfs_drop_snapshot

btrfs: fix missing error return in btrfs_drop_snapshot

commit e19182c0fff451e3744c1107d98f072e7ca377a0 upstream.

If btrfs_del_root fails in btrfs_drop_snapshot, we'll pick up the
error but then return 0 anyway due to mixing err and ret.

Fixes: 79787eaab4612 ("btrfs: replace many BUG_ONs with proper error handling")
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>