target: Add __exit annotation for module_exit functions

Inclues sbp_exit, fileio_module_exit, iblock_module_exit and
pscsi_module_exit.

Note: rd_module_exit() can not be annotated by __exit, becasue it is
called by target_core_init_configfs() which is annotated by __init.

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/sbp/sbp_target.c

@@ -2598,7 +2598,7 @@ static int __init sbp_init(void)
 	return 0;
 };
 
-static void sbp_exit(void)
+static void __exit sbp_exit(void)
 {
 	sbp_deregister_configfs();
 };