Generalize ffs_pages_remove() into vn_pages_remove().

Remove mapped pages for all dataset vnodes in zfs_rezget() using
new vn_pages_remove() to fix mmapped files changed by
zfs rollback or zfs receive -F.

PR: kern/160035, kern/156933
Reviewed by: kib, pjd
Approved by: re (kib)
MFC after: 1 week