MINOR: memprof: make in_memprof a bitfield instead of a counter

MINOR: memprof: make in_memprof a bitfield instead of a counter

It's not convenient to use it as it is now because it may only be
used to count passes via the memprof init code. Let's turn it to
a bitfield instead so that we can also check what we're doing there.
This is safe because all callers of memprof_init() check for the
bit being zero first so it's not reentrant.