bcachefs: Fix allocate -> self healing path

When we go to allocate and find taht a bucket in the freespace btree is
actually allocated, we're supposed to return nonzero to tell the
allocator to skip it.

This fixes an emergency read only due to a bucket/ptr gen mismatch - we
also don't return the correct bucket gen when this happens.

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