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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34377