malloc: fix race condition and other bugs in the no-mmu malloc

malloc: fix race condition and other bugs in the no-mmu malloc

Fixes multiple race conditions on mmb list. This was done by
making the mmb_heap_lock into a recursive lock and making the
regular heap_lock extend to cover the mmb heap handling.

Also move the new_mmb allocation up to before the mmb list is
iterated through to find the insertion point. When the mmb_heap
also runs out and needs to be extended when the regular heap is
just extended, the mmb list could be messed up.

Signed-off-by: Freeman Wang <xwang@ubicom.com>
Signed-off-by: Austin Foxley <austinf@cetoncorp.com>