MFC: r212443

MFC: r212443
This patch applies one of the two fixes suggested by
zack.kirsch at isilon.com for a race between nfsrv_freeopen()
and nfsrv_getlockfile() in the experimental NFS server that
he found during testing. Although nfsrv_freeopen() holds a
sleep lock on the lock file structure when called with
cansleep != 0, nfsrv_getlockfile() could still search the
list, once it acquired the NFSLOCKSTATE() mutex. I believe
that acquiring the mutex in nfsrv_freeopen() fixes the race.

git-svn-id: svn://svn.freebsd.org/base/stable/8@213191 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f