MFC r328266:

mlock(2): correct documentation for error conditions.

The man page is years out of date regarding errors. Our implementation _does_
allow unaligned addresses, and it _does_not_ check for negative lengths,
because the length is unsigned. It checks for overflow instead.

Update the tests accordingly.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D13826