nilfs2: insert explanations in gcinode file

The file gcinode.c gives buffer cache functions for on-disk blocks
moved in garbage collection.  Joern Engel has suggested inserting its
explanations in the source file (Message-ID:
<20080917144146.GD8750@logfs.org> and
<20080917224953.GB14644@logfs.org>).

This follows the comment.

Cc: Joern Engel <joern@logfs.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>