kernel: avoid pgoff overflow in remap_file_pages

[Eugene Teo <eugene () redhat com>]

Thomas Pollet reported an integer overflow issue in remap_file_pages(). 
While we are able to reproduce the issue, we are unable to find a 
security impact. If your views differ, do let us know.

https://bugzilla.redhat.com/show_bug.cgi?id=636383
http://git.kernel.org/linus/5ec1055aa5632dd7a8283cdb5fa9be3c535eaa06

Thanks, Eugene
--
main(i) { putchar(182623909 >> (i-1) * 5&31|!!(i<7)<<6) && main(++i); }