Reintegrate r250907 (previously reverted as r254754) plus MFS r254947,

Reintegrate r250907 (previously reverted as r254754) plus MFS r254947,
r255443:

r250907 (scottl):
  MFC r248830, r250027, r250409: Several fixes and improvements
  to sendfile()

r254947 (kib):
  NFS implementation of VOP_READ() sometimes upgrades the vnode lock,
  which causes drop of the shared lock and sleep for exclusive.  As
  result, busying of the page before the call to vn_rdwr() makes NFS
  code to wait for vnode lock while page is busy, which contradicts the
  proper order of vnode lock -> busy.

r255443 (des):
  Fix the length calculation for the final block of a sendfile(2)
  transmission which could be tricked into rounding up to the nearest
  page size, leaking up to a page of kernel memory.  [13:11]

Approved by:	re (delphij)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/releng/9.2@255456 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f