locks: fix two potential overflows in the lock delay code

With large numbers of CPUs, the calculation of the maximum lock delay
could overflow, leading to an unexpectedly low delay. In fact, the
maximum delay would calculate to 0 on systems with between 128 and
255 cores (inclusive). Also, when calculating the new delay in
lock_delay(), the delay would overflow if the old delay was >= 32,768.

This commit fixes these two overflows. It also updates the maximum
delay from 32,678 to SHRT_MAX.

Reviewed by: gallatin, jhb, mjg
Fixes: 6b8dd26e7c5f ("locks: convert delay times to u_short")
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D39372