btrfs: do not BUG_ON() on ENOMEM when dropping extent items for a range

If we get -ENOMEM while dropping file extent items in a given range, at
btrfs_drop_extents(), due to failure to allocate memory when attempting to
increment the reference count for an extent or drop the reference count,
we handle it with a BUG_ON(). This is excessive, instead we can simply
abort the transaction and return the error to the caller. In fact most
callers of btrfs_drop_extents(), directly or indirectly, already abort
the transaction if btrfs_drop_extents() returns any error.

Also, we already have error paths at btrfs_drop_extents() that may return
-ENOMEM and in those cases we abort the transaction, like for example
anything that changes the b+tree may return -ENOMEM due to a failure to
allocate a new extent buffer when COWing an existing extent buffer, such
as a call to btrfs_duplicate_item() for example.

So replace the BUG_ON() calls with proper logic to abort the transaction
and return the error.

Reported-by: syzbot+0b1fb6b0108c27419f9f@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/00000000000089773e05ee4b9cb4@google.com/
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.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/file.c

@@ -380,7 +380,10 @@ next_slot:
 						args->start - extent_offset,
 						0, false);
 				ret = btrfs_inc_extent_ref(trans, &ref);
-				BUG_ON(ret); /* -ENOMEM */
+				if (ret) {
+					btrfs_abort_transaction(trans, ret);
+					break;
+				}
 			}
 			key.offset = args->start;
 		}
@@ -467,7 +470,10 @@ delete_extent_item:
 						key.offset - extent_offset, 0,
 						false);
 				ret = btrfs_free_extent(trans, &ref);
-				BUG_ON(ret); /* -ENOMEM */
+				if (ret) {
+					btrfs_abort_transaction(trans, ret);
+					break;
+				}
 				args->bytes_found += extent_end - key.offset;
 			}