md/bitmap: separate out loading a bitmap from initialising the structures.

dm makes this distinction between ->ctr and ->resume, so we need to
too.

Also get the new bitmap_load to clear out the bitmap first, as this is
most consistent with the dm suspend/resume approach

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

diff --cc drivers/md/bitmap.c
Simple merge

diff --cc drivers/md/bitmap.h
Simple merge

diff --cc drivers/md/md.c
Simple merge