vfs: add VOP_DELAYED_SETSIZE() and related infrastructure

The change generalizes code that was initially developed for nfs client
to handle filesystems that needs to call vnode_pager_setsize() while
only owning the vnode lock shared.  Since vnode pager might need to trim
or extend the vnode vm_object' page queue, the vnode lock for the call
must be owned exclusive.  This is typical for filesystems with remote
authorative source of file attributes, like nfs/p9/fuse.

Handle the conflict by delaying the vnode_pager_setsize() to the next
vnode locking to avoid relock.  But if the next locking request is in
shared mode, lock it exclusively instead, perform the delayed
vnode_pager_setsize() call by doing VOP_DEFAULT_SETSIZE(), and then
downgrade to shared.

Filesystems that opt into the feature must provide the implementation of
VOP_DELAYED_SETSIZE() that actually calls vnode_pager_setsize(), and use
vn_delay_setsize() helper to mark the vnode as requiring the delay call.


nfsclient: convert to use vn_delayed_setsize()