iscsi_tcp: fix locking around iscsi sk user data

iscsi_tcp: fix locking around iscsi sk user data

commit 03adb5f91280b433c3685d0ee86b2e1424af3d88 upstream.

iscsi_sw_tcp_conn_restore_callbacks could have set
the sk_user_data field to NULL then iscsi_sw_tcp_data_ready
could read that and try to access the NULL pointer. This
adds some checks for NULL sk_user_data in the sk
callback functions and it uses the sk_callback_lock to
set/get that sk_user_data field.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>