ksmbd: send proper error response in smb2_tree_connect()

Currently, smb2_tree_connect doesn't send an error response packet on
error.

This causes libsmb2 to skip the specific error code and fail with the
following:
 smb2_service failed with : Failed to parse fixed part of command
 payload. Unexpected size of Error reply. Expected 9, got 8

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>