Fix bitmap_sfu() regression.

Fix bitmap_sfu() to shift by LG_BITMAP_GROUP_NBITS rather than
hard-coded 6 when using linear (non-USE_TREE) bitmap search.  In
practice this affects only 64-bit systems for which sizeof(long) is not
8 (i.e. Windows), since USE_TREE is defined for 32-bit systems.

This regression was caused by b8823ab0
(Use linear scan for small bitmaps).

This resolves #368.