memzone: improve zero-length reserve

memzone: improve zero-length reserve

Currently, reserving zero-length memzones is done by looking at
malloc statistics, and reserving biggest sized element found in those
statistics. This has two issues.

First, there is a race condition. The heap is unlocked between the
time we check stats, and the time we reserve malloc element for memzone.
This may lead to inability to reserve the memzone we wanted to reserve,
because another allocation might have taken place and biggest sized
element may no longer be available.

Second, the size returned by malloc statistics does not include any
alignment information, which is worked around by being conservative and
subtracting alignment length from the final result. This leads to
fragmentation and reserving memzones that could have been bigger but
aren't.

Fix all of this by using earlier-introduced operation to reserve
biggest possible malloc element. This, however, comes with a trade-off,
because we can only lock one heap at a time. So, if we check the first
available heap and find *any* element at all, that element will be
considered "the biggest", even though other heaps might have bigger
elements. We cannot know what other heaps have before we try and
allocate it, and it is not a good idea to lock all of the heaps at
the same time, so, we will just document this limitation and
encourage users to reserve memzones with socket id properly set.

Also, fixup unit tests to account for the new behavior.

Fixes: fafcc11985a2 ("mem: rework memzone to be allocated by malloc")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>