Pin lmkd for real

We pin lmkd in memory so that we don't take page faults (and thus
requisition memory) while we're in the process of responding to a
low-memory condition. mlockall(2) is the right primitive for this
pinning. Previously, we used the MCL_FUTURE flag to mlockall: used
this way, mlockall doesn't actually pin all pages in memory, since
MCL_FUTURE affects only the default flags for future mappings and
doesn't affect mapping already in existence at the time of the
mlockall call --- like the lmkd executable itself.

This patch adds the MCL_CURRENT flag, which also pins all pages
already mapped.

Test: code inspection
Change-Id: I4563959367a2f0a9cadc3ea41731b7f311326685