usr.sbin/makefs: fix use-after-free in read_mtree_keywords()

The st variable is used as a shortcut for &node->inode->st, but in one
branch just before the exit we update node->inode without changing st.

Reported by:	AddressSanitizer
Reviewed By:	emaste
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D28570

usr.sbin/makefs/mtree.c

@@ -783,6 +783,8 @@ read_mtree_keywords(FILE *fp, fsnode *node)
 			free(node->inode);
 			node->inode = curino;
 			node->inode->nlink++;
+			/* Reset st since node->inode has been updated. */
+			st = &node->inode->st;
 		}
 	}