Simplify UIO_SYSSPACE and UIO_NOCOPY paths in uiomove

Uiomove can only block when the segflag is UIO_USERSPACE,
otherwise we end up just doing a bcopy (or nothing) and
moving cursors. So only emit witness warnings and
set deadlock thread flags in the UIO_USERSPACE case.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D11489