MFC: r326544
Avoid the overhead of acquiring a lock in nfsrv_checkgetattr() when
there are no write delegations issued.

manu@ reported on the freebsd-current@ mailing list that there was
a significant performance hit in nfsrv_checkgetattr() caused by
the acquisition/release of a state lock, even when there were no
write delegations issued.
This patch add a count of outstanding issued write delegations to the
NFSv4 server. This count allows nfsrv_checkgetattr() to return without
acquiring any lock when the count is 0, avoiding the performance hit
for the case where no write delegations are issued.