cifs: Fix support for remount when not changing rsize/wsize

When remounting with the new mount API, we need to set
rsize and wsize to the previous values if they are not passed
in on the remount. Otherwise they get set to zero which breaks
xfstest 452 for example.

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>