btrfs: remove BUG_ON()'s in add_new_free_space()

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

btrfs: remove BUG_ON()'s in add_new_free_space()

At add_new_free_space() we have these BUG_ON()'s that are there to deal

with any failure to add free space to the in memory free space cache.

Such failures are mostly -ENOMEM that should be very rare. However there's

no need to have these BUG_ON()'s, we can just return any error to the

caller and all callers and their upper call chain are already dealing with

errors.

So just make add_new_free_space() return any errors, while removing the

BUG_ON()'s, and returning the total amount of added free space to an

optional u64 pointer argument.