Ensure that ta_pending doesn't overflow u_short by capping its value at USHRT_MAX.

If it overflows before the taskqueue can run, the task will be
re-added to the taskqueue and cause a loop in the task list.

Reported by: Arnaud Lacombe <lacombar@gmail.com>
Submitted by: Ryan Stone <rysto32@gmail.com>
Reviewed by: jhb
Approved by: re (kib)
MFC after: 1 day