lockmgr: shrink struct lock by 8 bytes on LP64

[mjg]

Summary

Currently the struct has a 4 byte padding stemming from 3 ints.

1. prio comfortably fits in short, unfortunately there is no dedicated type for it and plumbing it throughout the codebase is not worth it right now, instead an assert is added which covers also flags for safety
2. lk_exslpfail can in principle exceed u_short, but the count is already not considered reliable and it only ever gets modified straight to 0. In other words it can be incrementing with an upper bound of USHORT_MAX

With these in place struct lock shrinks from 48 to 40 bytes.

[kib]

When lk_exslpfail is saturated, shouldn't you always broadcast wakeup on the exclusive queue? It should be rare enough to not matter, but waking only one waiter there might cause hang IMO.

[mjg]

• add missing saturation checks

[kib]

sys/kern/kern_lock.c

357

Is this line too long now?

[mjg]

sys/kern/kern_lock.c

357

It is 84 if that's what you mean, but few lines below there is a 82 and in general the limit is rather soft at least in this file so I don't think this is a problem.