btrfs: do not BUG_ON() on failure to update inode when setting xattr

We are doing a BUG_ON() if we fail to update an inode after setting (or
clearing) a xattr, but there's really no reason to not instead simply
abort the transaction and return the error to the caller. This should be
a rare error because we have previously reserved enough metadata space to
update the inode and the delayed inode should have already been setup, so
an -ENOSPC or -ENOMEM, which are the possible errors, are very unlikely to
happen.

So replace the BUG_ON()s with a transaction abort.

CC: stable@vger.kernel.org # 4.9+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>