scsi: iscsi: iscsi_tcp: Fix null-ptr-deref while calling getpeername()

In the Linux kernel, the following vulnerability has been resolved:

scsi: iscsi: iscsi_tcp: Fix null-ptr-deref while calling getpeername()

Fix a NULL pointer crash that occurs when we are freeing the socket at the
same time we access it via sysfs.

The problem is that:

1. iscsi_sw_tcp_conn_get_param() and iscsi_sw_tcp_host_get_param() take
the frwd_lock and do sock_hold() then drop the frwd_lock. sock_hold()
does a get on the "struct sock".

2. iscsi_sw_tcp_release_conn() does sockfd_put() which does the last put
on the "struct socket" and that does __sock_release() which sets the
sock->ops to NULL.

3. iscsi_sw_tcp_conn_get_param() and iscsi_sw_tcp_host_get_param() then
call kernel_getpeername() which accesses the NULL sock->ops.

Above we do a get on the "struct sock", but we needed a get on the "struct
socket". Originally, we just held the frwd_lock the entire time but in
commit bcf3a2953d36 ("scsi: iscsi: iscsi_tcp: Avoid holding spinlock while
calling getpeername()") we switched to refcount based because the network
layer changed and started taking a mutex in that path, so we could no
longer hold the frwd_lock.

Instead of trying to maintain multiple refcounts, this just has us use a
mutex for accessing the socket in the interface code paths.