f2fs: avoid duplicated code by reusing f2fs_read_end_io

This patch tries to clean up code because part code of f2fs_read_end_io
and mpage_end_io are the same, so it's better to merge and reuse them.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>