[Bug 1920892] [NEW] locked memory allocation failure should fallback to unlocked memory

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When OVS is deployed inside of a LXD container the amount of locked
memory is limited by the configured system limits for the container -
until recently this was very small (64K) but was bumped to 16M and then
64M by recent changes into systemd.

OVS will attempt to lock current and future memory allocations on
startup - if this fails then subsequent memory allocations will not be
locked but its not a fatal error - the daemons can still run, but there
is a potential performance impact when memory contention occurs.

@64K this happened reliably the the ovs-vswitchd daemon would run
without locked memory.

At 16M and 64M this is less clear cut - the initial mlockall call
succeeds so the daemon runs with memory locking enabled for future
malloc etc.. calls - if one fails then the daemon will abort.  On a
modern server with many cores the amount of locked memory can quite
easily exceed the now higher limits inside of a container.

Outside of a container running as the root user the daemon has unlimited
access to locked memory (to the physical memory limits of the server).

Rather than completely disabling mlock when running in containers (which
would be one approach to avoid this issue) it could be better to
fallback to unlocked memory usage if the limit is hit.

mmap and related calls would set EAGAIN when the limit is hit.

** Affects: openvswitch (Ubuntu)
     Importance: Undecided
         Status: New

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locked memory allocation failure should fallback to unlocked memory
https://bugs.launchpad.net/bugs/1920892
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