[kernel] Fix frame allocator next-block bug

The frame allocator was causing page faults when exhausting the first
(well, last, because it starts from the end) block of free pages. Turns
out it was just incrementing instead of decrementing and thus running
off the end.

src/kernel/frame_allocator.cpp

@@ -32,7 +32,7 @@ bsf(uint64_t v)
 size_t
 frame_allocator::allocate(size_t count, uintptr_t *address)
 {
-	for (long i = m_count - 1; i >= 0; ++i) {
+	for (long i = m_count - 1; i >= 0; --i) {
 		frame_block &block = m_blocks[i];
 
 		if (!block.map1)