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

[dchagin]

Summary

Drop proc lock earlier, before copying user stuff.

Pointed out by: kib

[dchagin]

in the expectation:
ksiginfo_t *ksi
sigset_t *mask
both are on stack and do not need to proc lock

[kib]

sys/amd64/linux/linux_sysvec.c

480

Strictly speaking, you can unlock right before this line.

[dchagin]

sure, I left these lines together as a unit of code for sp,
now done.