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>
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 77ffa80..247b31e 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -1066,6 +1066,18 @@
 	/* reference all summary page */
 	while (segno < end_segno) {
 		sum_page = f2fs_get_sum_page(sbi, segno++);
+		if (IS_ERR(sum_page)) {
+			int err = PTR_ERR(sum_page);
+
+			end_segno = segno - 1;
+			for (segno = start_segno; segno < end_segno; segno++) {
+				sum_page = find_get_page(META_MAPPING(sbi),
+						GET_SUM_BLOCK(sbi, segno));
+				f2fs_put_page(sum_page, 0);
+				f2fs_put_page(sum_page, 0);
+			}
+			return err;
+		}
 		unlock_page(sum_page);
 	}