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

In the Linux kernel, the following vulnerability has been resolved:

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.