CIFS: check if pages is null rather than bv for a failed allocation

pages is being allocated however a null check on bv is being used
to see if the allocation failed. Fix this by checking if pages is
null.

Detected by CoverityScan, CID#1432974 ("Logically dead code")

Fixes: ccf7f408 ("CIFS: Add asynchronous context to support kernel AIO")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>