super-0.90: don't write bitmap larger than 60K

super-0.90: don't write bitmap larger than 60K

The 4K superblock can be as close as 64K from the end
of the device.  As the bitmap (with header) lives after
the superblock (with 0.90 metadata) there could be as
little as 60K of space.
So limit the bitmaps to 59.5K, and only write 60K including
the header.

The bug fixed here means that bitmaps cannot be created
on devices which are exact multiples of 64K in size

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