To date, mlockall(MCL_FUTURE) has had the unfortunate side effect of
blocking vm map entry and object coalescing for the calling process.
However, there is no reason that mlockall(MCL_FUTURE) should block
such coalescing. This change enables it.

Reviewed by: kib, markj
Tested by: pho
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D16413