bcachefs: Add printf format attribute to bch2_pr_buf()

This tells the compiler to check printf format strings, and catches a
few bugs.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_io.c

@@ -1069,7 +1069,7 @@ int bch2_btree_node_read_done(struct bch_fs *c, struct bch_dev *ca,
 
 			bch2_bkey_val_to_text(&buf, c, u.s_c);
 			btree_err(BTREE_ERR_FIXABLE, c, NULL, b, i,
-				  "invalid bkey %s: %s", buf, invalid);
+				  "invalid bkey %s: %s", buf.buf, invalid);
 			printbuf_exit(&buf);
 
 			btree_keys_account_key_drop(&b->nr, 0, k);

fs/bcachefs/super-io.c

@@ -1586,7 +1586,7 @@ void bch2_sb_to_text(struct printbuf *out, struct bch_sb *sb,
 
 	pr_buf(out, "Oldest version on disk:");
 	pr_tab(out);
-	pr_buf(out, "%u", bch2_metadata_versions[le16_to_cpu(sb->version_min)]);
+	pr_buf(out, "%s", bch2_metadata_versions[le16_to_cpu(sb->version_min)]);
 	pr_newline(out);
 
 	pr_buf(out, "Created:");

fs/bcachefs/util.h

@@ -282,7 +282,8 @@ static inline size_t printbuf_linelen(struct printbuf *buf)
 	return buf->pos - buf->last_newline;
 }
 
-void bch2_pr_buf(struct printbuf *out, const char *fmt, ...);
+void bch2_pr_buf(struct printbuf *out, const char *fmt, ...)
+	__attribute__ ((format (printf, 2, 3)));
 
 #define pr_buf(_out, ...) bch2_pr_buf(_out, __VA_ARGS__)