jffs2: Remove jffs2_gc_fetch_page and jffs2_gc_release_page

Merge these two helpers into the only callers to get rid of some
amazingly bad calling conventions.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Richard Weinberger <richard@nod.at>