super1 - do metadata IO in sector_size units.

super1 - do metadata IO in sector_size units.

If the sector size is > 512, we need to be more careful about
alignment.
The largest known sector size is 4096 and (fortunately) both the
superblock and (in many cases) the bitmap are 4096-byte aligned.
So there should be no data-overlap problems.

The exception is when the bitmap is squeezed into the 3K after the
superblock.  This arrangement cannot currently be supported on
4K sector-size devices.

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