fs/cifs/smb2pdu.c: fix buffer free in SMB2_ioctl_free
The 2nd buffer could be NULL even if iov_len is not zero. This can
trigger a panic when handling symlinks. It's easy to reproduce with
LTP fs_racer scripts[1] which are randomly craete/delete/link files
and dirs. Fix this panic by checking if the 2nd buffer is padding
before kfree, like what we do in SMB2_open_free.
Fixes: 2c87d6a94d16 ("cifs: Allocate memory for all iovs in smb2_ioctl") Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com> Signed-off-by: Steve French <stfrench@microsoft.com> Reviewed-by: Ronnie sahlberg <lsahlber@redhat.com>