vfs: Fix error handling in vn_fullpath_hardlink()

vn_fullpath_any_smr() will return a positive error number if the
caller-supplied buffer isn't big enough.  In this case the error must be
propagated up, otherwise we may copy out uninitialized bytes.

Reported by:	syzkaller+KMSAN
Reviewed by:	mjg, kib
MFC aftr:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30198

sys/kern/vfs_cache.c

@@ -3646,9 +3646,9 @@ vn_fullpath_hardlink(struct nameidata *ndp, char **retbuf, char **freebuf,
 		error = vn_fullpath_dir(vp, pwd->pwd_rdir, buf, retbuf, buflen,
 		    addend);
 		pwd_drop(pwd);
-		if (error != 0)
-			goto out_bad;
 	}
+	if (error != 0)
+		goto out_bad;
 
 	*freebuf = buf;