cifs: when extending a file with falloc we should make files not-sparse

as this is the only way to make sure the region is allocated.
Fix the conditional that was wrong and only tried to make already
non-sparse files non-sparse.

Cc: stable@vger.kernel.org
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>