mempool: reverse free bit semantic

This turns the free-bit flag into an alloc-bit flag effectively
reversing its semantic. This is to make further changes more natural
and easier to understand.

No need to clear the alloc bits at init time as they're located in .bss
and all clear already.

The code remains functionally equivalent after this change.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>