md: remove clumsy usage of do_sync_mapping_range from bitmap code

and replace with vfs_fsync which is much neater (but wasn't exported,
or even in existence at the time the code was written).

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NeilBrown <neilb@suse.de>

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