btrfs: remove pointless BUG_ON() when creating snapshot

When creating a snapshot we first check with btrfs_lookup_dir_item() if
there is a name collision in the parent directory and then return an error
if there's a collision. Then later on when trying to insert a dir item for
the snapshot we BUG_ON() if the return value is -EEXIST or -EOVERFLOW:

  static noinline int create_pending_snapshot(...)
  {
     (...)

     /* check if there is a file/dir which has the same name. */
     dir_item = btrfs_lookup_dir_item(...);
     (...)

     ret = btrfs_insert_dir_item(...);
     /* We have check then name at the beginning, so it is impossible. */
     BUG_ON(ret == -EEXIST || ret == -EOVERFLOW);
     if (ret) {
        btrfs_abort_transaction(trans, ret);
        goto fail;
     }

     (...)
  }

It's impossible to get the -EEXIST because we previously checked for a
potential collision with btrfs_lookup_dir_item() and we know that after
that no one could have added a colliding name because at this point the
transaction is in its critical section, state TRANS_STATE_COMMIT_DOING,
so no one can join this transaction to add a colliding name and neither
can anyone start a new transaction to do that.

As for the -EOVERFLOW, that can't happen as long as we have the extended
references feature enabled, which is a mkfs default for many years now.

In either case, the BUG_ON() is excessive as we can properly deal with
any error and can abort the transaction and jump to the 'fail' label,
in which case we'll also get the useful stack trace (just like a BUG_ON())
from the abort if the error is either -EEXIST or -EOVERFLOW.

So remove the BUG_ON().
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -1863,8 +1863,6 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	ret = btrfs_insert_dir_item(trans, &fname.disk_name,
 				    BTRFS_I(parent_inode), &key, BTRFS_FT_DIR,
 				    index);
-	/* We have check then name at the beginning, so it is impossible. */
-	BUG_ON(ret == -EEXIST || ret == -EOVERFLOW);
 	if (ret) {
 		btrfs_abort_transaction(trans, ret);
 		goto fail;