MFC r286970:

MFC r286970:

  Prevent ticks rollover from preventing vm_lowmem event

  Currently vm_pageout_scan() uses a ticks-based scheme to rate-limit
  the number of times that the vm_lowmem event will happen.  However
  if no events happen for long enough for ticks to roll over, this
  leaves us in a long window in which vm_lowmem events will not
  happen.

  Replace the use of ticks with time_t to prevent rollover from ever
  being an issue.

  Reviewed by:  ian
  MFC after:    3 weeks
  Sponsored by: EMC / Isilon Storage Division
  Differential Revision:        https://reviews.freebsd.org/D3439

git-svn-id: svn://svn.freebsd.org/base/stable/10@287922 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f