librbd: invalidate cache outside cache callback context

librbd: invalidate cache outside cache callback context

When shrinking an image, it's possible that the op flush callback
will be from within the cache callback context.  This would result
in a deadlock when attempting to re-lock the cache lock in order to
invalidate the cache.

Fixes: #11743
Backport: hammer
Signed-off-by: Jason Dillaman <dillaman@redhat.com>