NFS: suppress a build warning

struct sockaddr_storage * can safely be used as struct sockaddr *.
Suppress an "incompatible pointer type" warning.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>