mm/page-writeback.c: fix divide by zero in pos_ratio_polynom

mm/page-writeback.c: fix divide by zero in pos_ratio_polynom

It is possible for "limit - setpoint + 1" to equal zero, after getting
truncated to a 32 bit variable, and resulting in a divide by zero error.

Using the fully 64 bit divide functions avoids this problem.  It also
will cause pos_ratio_polynom() to return the correct value when
(setpoint - limit) exceeds 2^32.

Also uninline pos_ratio_polynom, at Andrew's request.

Signed-off-by: Rik van Riel <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Nishanth Aravamudan <[email protected]>
Cc: Luiz Capitulino <[email protected]>
Cc: Masayoshi Mizuma <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>