We replace 'size' by 'start'.  'start' means exactly the same as
'curr_offset - size', and the equivalence of the new code can be tested
based on this.  The difference is that 'start' will never be negative and
so can fit in a 'sector_t' while 'size' could be negative.

Also make curr_offset sector_t, as it should have been.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>