bcachefs: Fix for sb buffer being misaligned

On old kernels, kmalloc() may return an allocation that's not naturally
aligned - this resulted in a bug where we allocated a bio with not
enough biovecs. Fix this by using buf_pages().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super-io.c

@@ -203,8 +203,14 @@ int bch2_sb_realloc(struct bch_sb_handle *sb, unsigned u64s)
 	if (dynamic_fault("bcachefs:add:super_realloc"))
 		return -BCH_ERR_ENOMEM_sb_realloc_injected;
 
+	new_sb = krealloc(sb->sb, new_buffer_size, GFP_NOFS|__GFP_ZERO);
+	if (!new_sb)
+		return -BCH_ERR_ENOMEM_sb_buf_realloc;
+
+	sb->sb = new_sb;
+
 	if (sb->have_bio) {
-		unsigned nr_bvecs = DIV_ROUND_UP(new_buffer_size, PAGE_SIZE);
+		unsigned nr_bvecs = buf_pages(sb->sb, new_buffer_size);
 
 		bio = bio_kmalloc(nr_bvecs, GFP_KERNEL);
 		if (!bio)
@@ -216,11 +222,6 @@ int bch2_sb_realloc(struct bch_sb_handle *sb, unsigned u64s)
 		sb->bio = bio;
 	}
 
-	new_sb = krealloc(sb->sb, new_buffer_size, GFP_NOFS|__GFP_ZERO);
-	if (!new_sb)
-		return -BCH_ERR_ENOMEM_sb_buf_realloc;
-
-	sb->sb = new_sb;
 	sb->buffer_size = new_buffer_size;
 
 	return 0;