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 bcf3a295 ("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.

Link: https://lore.kernel.org/r/20220907221700.10302-1-michael.christie@oracle.com


Fixes: bcf3a295 ("scsi: iscsi: iscsi_tcp: Avoid holding spinlock while calling getpeername()")
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>