[SCSI] iscsi_tcp: fix locking around iscsi sk user data

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>