CIFS: Fix variable types in cifs_iovec_read/write (try #2)

Variable 'i' should be unsigned long as it's used in circle with num_pages,
and bytes_read/total_written should be ssize_t according to return value.

Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>