TOMOYO: Simplify garbage collector.

When TOMOYO started using garbage collector at commit 847b173e "TOMOYO: Add
garbage collector.", we waited for close() before kfree(). Thus, elements to be
kfree()d were queued up using tomoyo_gc_list list.

But it turned out that tomoyo_element_linked_by_gc() tends to choke garbage
collector when certain pattern of entries are queued.

Since garbage collector is no longer waiting for close() since commit 2e503bbb
"TOMOYO: Fix lockdep warning.", we can remove tomoyo_gc_list list and
tomoyo_element_linked_by_gc() by doing sequential processing.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>