- Avoid including sys/proc.h in linux_vdso_gtod.c. It's not needed, but the implicit inclusion of sys/param.h via sys/ucred.h->bsm/audit.h was bringing in some required definitions.
- Include a couple of required headers: sys/time.h (for struct bintime), and limits.h (for INT_MAX).
- Move some helpers from linux.h, which depend on sys/param.h for NODEV, to the one CU where they're actually used.
No functional change intended.