vm: stop taking proc lock in mmap to satisfy racct if it is disabled

Limits can be safely obtained with lim_cur from the thread. racct is compiled
in but disabled by default. Note that racct enablement is a boot-only tunable.

This eliminates second most common place of taking the lock while pkg building.

While here don't take the lock in mlockall either.

Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17210