cifs: Deal with some warnings from W=1

Deal with some warnings generated from make W=1:

 (1) Add/remove/fix kerneldoc parameters descriptions.

 (2) Turn cifs' rqst_page_get_length()'s banner comment into a kerneldoc
     comment.  It should probably be prefixed with "cifs_" though.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>