ext2: fix ext2_splice_branch() comments

There is no argument named @chain in ext2_splice_branch, remove references
to it.

Signed-off-by: Qinghuang Feng <qhfeng.kernel@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>