rmlock: Micro-optimize read locking

Use get_pcpu() instead of an open-coded pcpu_find(td->td_oncpu). This
eliminates some memory accesses and results in a shorter instruction
sequence. Note that get_pcpu() didn't exist when rmlocks were added.

Reviewed by: jah, mjg
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c84bb8cd771ce4bed58152e47a32dda470bef23a)