cifs: fix revalidation test in cifs_llseek()

This test is always true so it means we revalidate the length every
time, which generates more network traffic.  When it is SEEK_SET or
SEEK_CUR, then we don't need to revalidate.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>