CVE request - kernel: nfsd: fix vm overcommit crash

[Eugene Teo <eugeneteo () kernel sg>]

"knfsd crashes if you are using it to export shmemfs objects and run 
strict overcommit. In this situation the current->mm based modifier to 
the overcommit goes through a NULL pointer.

We could simply check for NULL and skip the modifier but we've caught 
other real bugs in the past from mm being NULL here - cases where we did 
need a valid mm set up (eg the exec bug in 2005).

To preserve the checks and get the logic we want shuffle the checking 
around and add a new helper to the vm_ security wrappers

Also fix a current->mm reference in nommu that should use the passed mm"

Upstream commit:
nfsd: fix vm overcommit crash
http://git.kernel.org/linus/731572d39fcd3498702eda4600db4c43d51e0b26

Reference:
[PATCH] knfsd: add nfs-export support to tmpfs
http://git.kernel.org/linus/91828a405ae454a9503c41a7744f6ff877a80714
https://bugzilla.redhat.com/show_bug.cgi?id=595970

Backtrace (-rt kernel):
https://bugzilla.redhat.com/show_bug.cgi?id=595970#c1

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