bcachefs: fix odebug warn and lockdep splat due to on-stack rhashtable

bcachefs: fix odebug warn and lockdep splat due to on-stack rhashtable

Guenter Roeck reports a lockdep splat and DEBUG_OBJECTS_WORK related
warning when bch2_copygc_thread() initializes its rhashtable. The
lockdep splat relates to a warning print caused by the fact that the
rhashtable exists on the stack but is not annotated as so. This is
something that could be addressed by INIT_WORK_ONSTACK(), but
rhashtable doesn't expose that control and probably isnt worth the
churn for just one user. Instead, dynamically allocate the
buckets_in_flight structure and avoid the splat that way.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>