Pick 50b473c8839f5408df179bdf6f2b3fd2cf5c3b2f from upstream:

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