munmap.2: Remove EINVAL for negative len

len is unsigned (it is size_t), so cannot be negative.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit fab411c4fd5224e3dd44e0eb288d60b27480e2d1)
(cherry picked from commit e2cbfa1f5045019d34eb0091db7755c151ea06d5)