f2fs: code cleanup and simplify in func {find/add}_gc_inode

This patch simplifies list operations in find_gc_inode and add_gc_inode.
Just simple code cleanup.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
[Jaegeuk Kim: add description]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>