bcachefs: remove bucket_gens btree keys on device removal

bcachefs: remove bucket_gens btree keys on device removal

If a device has keys in the bucket_gens btree associated with its
buckets and is removed from a bcachefs volume, fsck will complain
about the presence of keys associated with an invalid device index.
A repair removes the associated keys and restores correctness.
Update bch2_dev_remove_alloc() to remove device related keys at
device removal time to avoid the problem.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>