md: fix max sectors calculation for super 1.0

To grow size of super 1.0 raid array, it is necessary to check the device
max usable size.

Now it uses rdev->sectors for max usable size. If one disk is 500G and the
raid device only uses the 100GB of this disk. rdev->sectors can't tell the
real max usable size. The max usable size should be

dev_size-(superblock_size+bitmap_size+badblock_size).

Also, remove unnecessary sb_start update in super_1_rdev_size_change().

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>