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.
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.

MFC after:	2 weeks

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