Fix deadlock in leak detector

Commit 65afd075 that fixed mincore()
exposed a deadlock in the leak detector, caused by two threads taking
two locks in opposite order:

Thread 1:  malloc() does alloc_tracker::remember(). This takes the tracker
   lock and calls backtrace() calling mincore() which takes the
   vma_list_mutex.

Thread 2: mmap() does mmu::allocate() which takes the vma_list_mutex and
   then through mmu::populate::small_page calls memory::alloc_page() which
   calls alloc_tracker::remember() and takes the tracker lock.

This patch fixes this deadlock: alloc_tracker::remember() will now drop its
lock while running backtrace(), as the lock is only needed to protect the
allocations[] array. We need to retake the lock after backtrace() completes,
to copy the backtrace back to the allocations[] array.

Previously, the lock's depth was also (ab)used for avoiding nested
allocation tracking (e.g., tracking of memory allocation done inside
backtrace() itself), but now that backtrace() is run without the lock,
we need a different mechanism - a per-thread "in_tracker" flag, which
is turned on inside the alloc_tracker::remember()/forget() methods.