md/bitmap: prepare for storing write-intent-bitmap via dm-dirty-log.

This allows md/raid5 to fully work as a dm target.

Normally md uses a 'filemap' which contains a list of pages of bits
each of which may be written separately.
dm-log uses and all-or-nothing approach to writing the log, so
when using a dm-log, ->filemap is NULL and the flags normally stored
in filemap_attr are stored in ->logattrs instead.

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.h
Simple merge