Make it possible to call VOP_PUTPAGES with shared vnode lock instead

Make it possible to call  VOP_PUTPAGES with shared vnode lock instead
of insisting an exclusive lock.

When VFS_LOCK_DEBUG is enabled and when the underlying file system is
capable of MNT_SHARED_WRITES,  a failed assertion may lead to a panic
because when calling fsync(2),  a.k.a.  sys_fsync() over a vnode with
mmap'ed page,  it will grab a shared lock when underlying file system
is capable,  then call vm_object_page_clean that eventually proceeded
to  vnode_pager_putpages()   that  tries  to do  VOP_PUTPAGES,  where
VOP_PUTPAGES_APV  generated from the declaration asserts the vnode is
always exclusively locked by caller.

This is a direct commit to stable/8 because a majority portion of the
original changeset 257899 (kib), which main goal was an optimization,
was omitted.   We saw the panic independently in a FreeNAS bug ticket
(#4578).

git-svn-id: svn://svn.freebsd.org/base/stable/8@263376 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f