mem: exclude unused memory from core dump

mem: exclude unused memory from core dump

Currently, even though memory is mapped with PROT_NONE, this does not
cause it to be excluded from core dumps. This is counter-productive,
because in a lot of cases, this memory will go unused (e.g. when the
memory subsystem preallocates VA space but hasn't yet mapped physical
pages into it).

Use `madvise()` call with MADV_DONTDUMP/MADV_NOCORE to exclude the
unused memory from being dumped.

Signed-off-by: Li Feng <fengli@smartx.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>