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>