ufs: sector_t cannot be negative

unsigned i_block,fragment cannot be negative.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Evgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>