malloc: avoid padding elements on page deallocation

malloc: avoid padding elements on page deallocation

Currently, when deallocating pages, malloc will fixup other
elements' headers if there is not enough space to store a full
element in leftover space. This leads to race conditions because
there are some functions that check for pad size with an unlocked
heap, expecting pad size to be constant.

Fix it by being more conservative and only freeing pages when
there is enough space before and after the page to store a free
element.

Fixes: 1403f87d4fb8 ("malloc: enable memory hotplug support")

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