d_path: prepend_path(): lift resetting b in case when we'd return 3 out of loop

preparation to extracting the loop into helper (next commit)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/d_path.c

@@ -115,7 +115,6 @@ static int prepend_path(const struct path *path,
 
 			/* Escaped? */
 			if (dentry != mnt->mnt.mnt_root) {
-				b = *p;
 				error = 3;
 				break;
 			}
@@ -156,6 +155,9 @@ static int prepend_path(const struct path *path,
 	}
 	done_seqretry(&mount_lock, m_seq);
 
+	if (unlikely(error == 3))
+		b = *p;
+
 	if (b.len == p->len)
 		prepend(&b, "/", 1);