Add an argument to nfsvno_getattr() in the experimental

Add an argument to nfsvno_getattr() in the experimental
NFS server, so that it can avoid calling VOP_ISLOCKED()
when the vnode is known to be locked. This will allow
LK_SHARED to be used for these cases, which happen to
be all the cases that can use LK_SHARED. This does not
fix any bug, but it reduces the number of calls to
VOP_ISLOCKED() and prepares the code so that it can be
switched to using LK_SHARED in a future patch.

Reviewed by:	kib
MFC after:	2 weeks