btrfs: don't WARN if we abort a transaction with EROFS

If we got some sort of corruption via a read and call
btrfs_handle_fs_error() we'll set BTRFS_FS_STATE_ERROR on the fs and
complain.  If a subsequent trans handle trips over this it'll get EROFS
and then abort.  However at that point we're not aborting for the
original reason, we're aborting because we've been flipped read only.
We do not need to WARN_ON() here.

CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>