Set commit properly for FreeBSD w/ overcommit.
When overcommit is enabled, commit needs to be set when doing mmap(). The
regression was introduced in f80c97e.
This fixes 'retain:true'.
Discussed with: jasone
Obtained from: Qi Wang <interwq at gwu dot edu>
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
086165ecb5