mm/mempolicy.c: use match_string() helper to simplify the code

mm/mempolicy.c: use match_string() helper to simplify the code

match_string() returns the index of an array for a matching string, which
can be used intead of open coded implementation.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: zhong jiang <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>