kernel - Close possible hole in hammer inval code

kernel - Close possible hole in hammer inval code

* Do not use FINDBLK_TEST, instead have findblk() return a locked buffer
  cache buffer and deal with it from there.  While the original code
  should have been ok (it would getblk() the buffer cache in either case),
  it depended on certain MP race characteristics that might not hold so
  don't take any chances.

* This does not fix any known issues but removes some uncertainty.