btrfs: don't double unlock on error in btrfs_punch_hole

If we have an error writing out a delalloc range in
btrfs_punch_hole_lock_range we'll unlock the inode and then goto
out_only_mutex, where we will again unlock the inode.  This is bad,
don't do this.

Fixes: f27451f2 ("Btrfs: add support for fallocate's zero range operation")
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>