Remove bogus use of useracc() in (clock_)nanosleep.

There's no point in pre-checking that we can access the user's rmtp
pointer before we do it in copyout().

While here, improve style(9) compliance.

Reviewed by: imp
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24409