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/segment.c b/fs/f2fs/segment.c
index 787e26c..876182a 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -2429,6 +2429,7 @@
 	__next_free_blkoff(sbi, curseg, 0);
 
 	sum_page = f2fs_get_sum_page(sbi, new_segno);
+	f2fs_bug_on(sbi, IS_ERR(sum_page));
 	sum_node = (struct f2fs_summary_block *)page_address(sum_page);
 	memcpy(curseg->sum_blk, sum_node, SUM_ENTRY_SIZE);
 	f2fs_put_page(sum_page, 1);
@@ -3904,6 +3905,8 @@
 
 			se = &sit_i->sentries[start];
 			page = get_current_sit_page(sbi, start);
+			if (IS_ERR(page))
+				return PTR_ERR(page);
 			sit_blk = (struct f2fs_sit_block *)page_address(page);
 			sit = sit_blk->entries[SIT_ENTRY_OFFSET(sit_i, start)];
 			f2fs_put_page(page, 1);