cifs: zero-range does not require the file is sparse

Remove the conditional to fail zero-range if the file is not flagged as sparse.
You can still zero out a range in SMB2 even for non-sparse files.

Tested with stock windows16 server.

Fixes 5 xfstests (033, 149, 155, 180, 349)

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>