malloc: fix ASan handling for unmapped memory

malloc: fix ASan handling for unmapped memory

Currently, when we free previously allocated memory, we mark the area as
"freed" for ASan purposes (flag 0xfd). However, sometimes, freeing a
malloc element will cause pages to be unmapped from memory and re-backed
with anonymous memory again. This may cause ASan's "use-after-free"
error down the line, because the allocator will try to write into
memory areas recently marked as "freed".

To fix this, we need to mark the unmapped memory area as "available",
and fixup surrounding malloc element header/trailers to enable later
malloc routines to safely write into new malloc elements' headers or
trailers.

Bugzilla ID: 994
Fixes: 6cc51b1293ce ("mem: instrument allocator for ASan")
Cc: stable@dpdk.org
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>