HAMMER VFS - Fix inconsistent atime updates on read()

HAMMER VFS - Fix inconsistent atime updates on read()

* HAMMER's optimization of the read() path to improve concurrency broke
  atime updates in the cache case.  There was a big XXX comment in the
  code about it.

* Fix the issue in a way that avoids acquiring the fs_token in the cache
  case.  If the ATIME flag is already set all we need to do is update
  ip->ino_data.atime.  If it is not set we have to get the fs_token and
  go through the normal transaction.

Reported-by: marino