Stop using shared locks for nfs vop locks.
Stop using shared locks for nfs vop locks.

The reason this was done was to avoid a race to the root when an
NFS server went down.  However a semi-recent change to the way that
the kernel's lookup() routine traverses mount points prevents this.

Rev 1.39 of vfs_lookup.c changed the ordering of locks such that we
aquire a shared lock on the mount point being accessed and then drop
the directory vnode lock before requesting the target lock.

With that in place we no longer need shared locks for NFS to prevent
race to the root lockups.

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