CIFS: fix oops in cifs_lookup during net boot

CIFS: fix oops in cifs_lookup during net boot

commit 8e6c0332d5032aef2d3bc8f41771f999112c8c66 upstream.

Fixes bugzilla.kernel.org bug number 14641

Lookup called during network boot (network root filesystem
for diskless workstation) has case where nd is null in
lookup.  This patch fixes that in cifs_lookup.

(Shirish noted that 2.6.30 and 2.6.31 stable need the same check)

Signed-off-by: Shirish Pargaonkar <shirishp@us.ibm.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Tested-by:  Vladimir Stavrinov <vs@inist.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>