target: put struct target_fabric_configfs on a diet

Remove all fields that are either unused or can be replaced by trivially
following pointers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/target/target_core_configfs.c
Simple merge

diff --cc drivers/target/target_core_fabric_configfs.c
Simple merge

diff --cc include/target/target_core_configfs.h
Simple merge