Add sparse file support to SMB2/SMB3 mounts

Many Linux filesystes make a file "sparse" when extending
a file with ftruncate. This does work for CIFS to Samba
(only) but not for SMB2/SMB3 (to Samba or Windows) since
there is a "set sparse" fsctl which is supposed to be
sent to mark a file as sparse.

This patch marks a file as sparse by sending this simple
set sparse fsctl if it is extended more than 2 pages.
It has been tested to Windows 8.1, Samba and various
SMB2/SMB3 servers which do support setting sparse (and
MacOS which does not appear to support the fsctl yet).
If a server share does not support setting a file
as sparse, then we do not retry setting sparse on that
share.

The disk space savings for sparse files can be quite
large (even more significant on Windows servers than Samba).

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Shirish Pargaonkar <spargaonkar@suse.com>

fs/cifs/smb2pdu.c

@@ -1224,7 +1224,9 @@ SMB2_ioctl(const unsigned int xid, struct cifs_tcon *tcon, u64 persistent_fid,
 
 	cifs_dbg(FYI, "SMB2 IOCTL\n");
 
-	*out_data = NULL;
+	if (out_data != NULL)
+		*out_data = NULL;
+
 	/* zero out returned data len, in case of error */
 	if (plen)
 		*plen = 0;