md/bitmap: add bitmap_resize function to allow bitmap resizing.

md/bitmap: add bitmap_resize function to allow bitmap resizing.

This function will allocate the new data structures and copy
bits across from old to new, allowing for the possibility that the
chunksize has changed.

Use the same function for performing the initial allocation
of the structures.  This improves test coverage.

Signed-off-by: NeilBrown <neilb@suse.de>