swapoff(2): add a SWAPOFF_FORCE flag

The flag requests skipping the heuristic which tries to avoid leaving
system with more allocated memory than available from RAM and remanining
swap.

Reviewed by: markj
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33165