CVE request - kernel: cifs: Fix a kernel BUG with remote OS/2 server

[Eugene Teo <eugeneteo () kernel sg>]

"This was known to trigger with a OS/2 server. The server sets 
pSMBr->CountHigh to a incorrect value even in case of normal writes. 
This results in 'nbytes' being computed wrongly and triggers a kernel 
BUG at mm/filemap.c.

    void iov_iter_advance(struct iov_iter *i, size_t bytes)
    {
            BUG_ON(i->count < bytes);    <--- BUG here

Why the server is setting 'CountHigh' is not clear but only does so 
after writing 64k bytes. Though this looks like the server bug, the 
client side crash may not be acceptable.

The workaround is to mask off high 16 bits if the number of bytes 
written as returned by the server is greater than the bytes requested by 
the client."

https://bugzilla.redhat.com/show_bug.cgi?id=608583
http://git.kernel.org/linus/6513a81e9325d712f1bfb9a1d7b750134e49ff18

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