client: more precise cap trimming

Two fixes:
 * Client would unlink everything it could, instead of just
   meeting its goal, because caps.size() doesn't change until
   dentries are cleaned up later.  Take account of the trimmed
   count in the while() condition to fix that.
 * Don't count the root ino as trimmed, as although it has no
   dentries (of course), we will never give up the cap.

With this change, the client will now precisely achieve the number
of caps requested in CEPH_SESSION_RECALL_STATE messages.

Signed-off-by: John Spray <john.spray@redhat.com>