From: William Lee Irwin III <wli@holomorphy.com>

Chang bitmap_shift_left()/bitmap_shift_right() to have O(1) stackspace
requirements.

Given zeroed tail preconditions these implementations satisfy zeroed tail
postconditions, which makes them compatible with whatever changes from Paul
Jackson one may want to merge in the future.  No particular effort was
required to ensure this.

A small (but hopefully forgiveable) cleanup is a spelling correction:
s/bitmap_shift_write/bitmap_shift_right/ in one of the kerneldoc comments.

The primary effect of the patch is to remove the MAX_BITMAP_BITS
limitation, so restoring the NR_CPUS to be limited only by stackspace and
slab allocator maximums.  They also look vaguely more efficient than the
current code, though as this was not done for performance reasons, no
performance testing was done.