[SCSI] target: Add __init/__exit annotation for target_core_[init,exit]_configfs

This patch fixes the follownig section mismatch warning:

WARNING: vmlinux.o(.text+0x21617a): Section mismatch in reference from the
function target_core_init_configfs() to the function .init.text:rd_module_init()

The function target_core_init_configfs() references the function __init
rd_module_init().  This is often because target_core_init_configfs() lacks
a __init annotation or the annotation of rd_module_init is wrong.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>