[CIFS] cifs: fix oops on mount when CONFIG_CIFS_DFS_UPCALL is enabled

simple "mount -t cifs //xxx /mnt" oopsed on strlen of options
http://kerneloops.org/guilty.php?guilty=cifs_get_sb&version=2.6.25-release&start=16711 \
68&end=1703935&class=oops

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>