Fix interface between nfsclient and vnode pager.

Make the nfsclient always call vnode_pager_setsize() with the vnode
exclusively locked. This ensures that page fault always can find the
backing page if the object size check succeeded. Set VV_VMSIZEVNLOCK
flag on NFS nodes.

The main offender breaking the interface in nfsclient is
nfs_loadattrcache(), which is used whenever server responded with
updated attributes, which can happen on non-changing operations as
well. Also, iod threads only have buffers locked (and even that is
LK_KERNPROC), but they still may call nfs_loadattrcache() on RPC
response.

Instead of immediately calling vnode_pager_setsize() if server
response indicated changed file size, but the vnode is not exclusively
locked, set a new node flag NVNSETSZSKIP. When the vnode exclusively
locked, or when we can temporary upgrade the lock to exclusive, call
vnode_pager_setsize(), by providing the nfsclient VOP_LOCK() implementation.

Tested by: pho
Discussed with: rmacklem
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D21883