nanosleep: plug a kernel memory disclosure

nanosleep() updates rmtp on EINVAL. In that case, kern_nanosleep()
has not updated rmt, so sys_nanosleep() updates the user-space rmtp
by copying garbage from its stack frame. This is not only a kernel
memory disclosure, it's also not POSIX-compliant. Fix it to update
rmtp only on EINTR.

Reviewed by: jilles (via D10020), dchagin
MFC after: 3 days
Security: possibly
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D10044