smb: client: fix potential NULL deref in parse_dfs_referrals()

[ Upstream commit 92414333eb375ed64f4ae92d34d579e826936480 ]

If server returned no data for FSCTL_DFS_GET_REFERRALS, @dfs_rsp will
remain NULL and then parse_dfs_referrals() will dereference it.

Fix this by returning -EIO when no output data is returned.

Besides, we can't fix it in SMB2_ioctl() as some FSCTLs are allowed to
return no data as per MS-SMB2 2.2.32.

Fixes: 9d49640a ("CIFS: implement get_dfs_refer for SMB2+")
Cc: stable@vger.kernel.org
Reported-by: Robert Morris <rtm@csail.mit.edu>
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>