mempool: Fix bit pointer state for N_MAX > 31

When a mempool is created with a large number of maximum-size blocks,
the logic for initializing max_inline_level (i.e. when to union the
bitmask with the pointer and when to use the pointer directly) was
wrong.  The default state was "zero", which implies that level 0
should be inlined, but that's wrong with >32 base blocks.
Additionally, the type was unsigned, making the "level zero is a
pointer" situation impossible to represent.

Fixes #6727

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>