Hello,

This patch fixes a regression in the size of bitmap_head that resulted
from the removal of all the #ifdef GATHER_STATISTICS tests.

Instead of a pointer to a descriptor, this patch gives each bitmap an
integer that is the index of the bitmap_descriptor.

Bootstrapped&tested on powerpc64-unknown-linux-gnu, and built on the
same platform with memory statistics gathering enabled.
OK for trunk?

Ciao!
Steven