nfsd: use i_wrlock instead of rcu for nfsdfs i_private

synchronize_rcu() gets called multiple times each time a client is
destroyed.  If the laundromat thread has a lot of clients to destroy,
the delay can be noticeable.  This was causing pynfs test RENEW3 to
fail.

We could embed an rcu_head in each inode and do the kref_put in an rcu
callback.  But simplest is just to take a lock here.

(I also wonder if the laundromat thread would be better replaced by a
bunch of scheduled work or timers or something.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>