fs/cifs: fix uninitialised variable warnings

In some error conditions, resp_buftype can be passed uninitialised to
free_rsp_buf(), potentially resulting in a spurious debug message.
If resp_buftype randomly had the value 1 (CIFS_SMALL_BUFFER) then this
would log a debug message.
The rsp pointer is initialised to NULL so there is no other side-effect.

Detected by CoverityScan, CID 1438585 ("Uninitialized scalar variable")
Detected by CoverityScan, CID 1438667 ("Uninitialized scalar variable")
Detected by CoverityScan, CID 1438764 ("Uninitialized scalar variable")

Signed-off-by: Garry McNulty <garrmcnu@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>