mem: improve best-effort allocation

mem: improve best-effort allocation

Previously, when using non-exact allocation, we were requesting
N pages to be allocated, but allowed the memory subsystem to
allocate less than requested. However, we were still expecting
to see N contigous free pages in the memseg list.

This presents a problem because there is no way to try and
allocate as many pages as possible, even if there isn't
enough contiguous free entries in the list.

To address this, use the new "find biggest" fbarray API's when
allocating non-exact number of pages. This way, we will first
check how many entries in the list are actually available, and
then try to allocate up to that number.

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