linux: reduce differences between rt_sendsig() and sendsig()

This makes it easier to compare the two. This involves moving
the mutex slightly lower down, but there should be no functional
changes.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30541

(cherry picked from commit 135dd0cab51e89739416d8f924ea984e364a110e)