malloc: allow reserving biggest element

malloc: allow reserving biggest element

Add an internal-only function to allocate biggest element from
the heap. Nominally, it supports SOCKET_ID_ANY as its socket
argument, but it's essentially useless because other sockets
will only be allocated from if the entire heap on current or
specified socket is busy.

Still, asking to reserve a biggest element will allow fixing
race condition in memzone reserve that has been there for a
long time.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>