cifs: do not include page data when checking signature

cifs: do not include page data when checking signature

On async reads, page data is allocated before sending.  When the
response is received but it has no data to fill (e.g.
STATUS_END_OF_FILE), __calc_signature() will still include the pages in
its computation, leading to an invalid signature check.

This patch fixes this by not setting the async read smb_rqst page data
(zeroed by default) if its got_bytes is 0.

This can be reproduced/verified with xfstests generic/465.

Cc: <[email protected]>
Signed-off-by: Enzo Matsumiya <[email protected]>
Reviewed-by: Paulo Alcantara (SUSE) <[email protected]>
Signed-off-by: Steve French <[email protected]>