f2fs: avoid f2fs_bug_on if f2fs_get_meta_page_nofail got EIO

This patch avoids BUG_ON when f2fs_get_meta_page_nofail got EIO during
xfstests/generic/475.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>