bcachefs: Kill bch2_invalidate_bucket()

bcachefs: Kill bch2_invalidate_bucket()

This patch is working towards eventually getting rid of the in memory
struct bucket, and relying only on the btree representation.

Since bch2_invalidate_bucket() was only used for incrementing gens, not
invalidating cached data, no other counters were being changed as a side
effect - meaning it's safe for the allocator code to increment the
bucket gen directly.

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