nfsd: Always lock state exclusively.

It used to be the case that state had an rwlock that was locked for write
by downgrades, but for read for upgrades (opens). Well, the problem is
if there are two competing opens for the same state, they step on
each other toes potentially leading to leaking file descriptors
from the state structure, since access mode is a bitmap only set once.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>