lockmgr: shrink struct lock by 8 bytes on LP64
Currently the struct has a 4 byte padding stemming from 3 ints.
- 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
- 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 USHRT_MAX
With these in place struct lock shrinks from 48 to 40 bytes.
Reviewed by: kib (previous version)
Differential Revision: https://reviews.freebsd.org/D28680