dm thin metadata: remove in-core 'read_only' flag

Leverage the block manager's read_only flag instead of duplicating it;
access with new dm_bm_is_read_only() method.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc drivers/md/dm-thin-metadata.c
Simple merge

diff --cc drivers/md/persistent-data/dm-block-manager.c
Simple merge

diff --cc drivers/md/persistent-data/dm-block-manager.h
Simple merge