linux(4): Microoptimize rt_sendsig() on amd64.

Drop proc lock earlier, before copying user stuff.

Pointed out by: kib
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38326
MFC after: 1 week

(cherry picked from commit 6ad07a4b2bdf0856545ff8495a7b3396695814fa)