md: collect bitmap-specific fields into one structure.

In preparation for making bitmap fields configurable via sysfs,
start tidying up by making a single structure to contain the
configuration fields.

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

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

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

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