FBSD, DFBSD (future) vnode_pager_setsize updates

FBSD, DFBSD (future)  vnode_pager_setsize updates

Based on review of bundled filesystems on FBSD and DFBSD,
call vnode_pager_setsize in three unhandled cases (getattr,
setattr, and io growing a file;  truncation was handled
correctly already).  Following up on a suggestion by Ben
Kaduk.

Change-Id: I05b61c26402bb8ab49ad74b71af2d02b439f959f
Reviewed-on: http://gerrit.openafs.org/2621
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 8334b40376f4b820e5f08e0a1f477be7c04c5cc8)
Reviewed-on: http://gerrit.openafs.org/2681