munmap.2: Unaligned addresses do not return error

We previously claimed that non-page-aligned addresses would return
EINVAL, but the address is in fact rounded down to the page boundary.

Reported by: Harald Eilertsen <haraldei@anduin.net>
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Fixes: dabee6fecc67 ("kern_descrip.c: add fdshare()/fdcopy()")
Differential Revision: https://reviews.freebsd.org/D48465