nfsd: document callback_wq serialization of callback code

nfsd: document callback_wq serialization of callback code

The callback code relies on the fact that much of it is only ever called
from the ordered workqueue callback_wq, and this is worth documenting.

Reported-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>