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