bcachefs: kill btree_trans_too_many_iters() in bch2_bucket_alloc_freelist()

When we're called via
trans commit -> btree split -> allocator

We may have already arbitrarily many btree_paths, for the transaction
commit we're trying to do; when this happens, the
btree_trans_too_many_iters() call causes us to livelock.

Since the allocator calls btree_iter_dontneed to release paths as it
iterates, this shouldn't cause any problems.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>