md: Fix types in sb writer

Page->index is a pgoff_t and multiplying could cause overflows on a
32-bit architecture. In the sb writer, this is used to calculate and
verify the sector being used, and is multiplied by a sector value. Using
sector_t will cast it to a u64 type and is the more appropriate type for
the unit. Additionally, the integer size unit is converted to a sector
unit in later calculations, and is now corrected to be an unsigned type.

Finally, clean up the calculations using variable aliases to improve
readabiliy.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jon Derrick <jonathan.derrick@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230224183323.638-3-jonathan.derrick@linux.dev