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

[arichardson]

Summary

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.

Not sure which of the two approaches is preferable

Reported by: AddressSanitizer

Test Plan

ASan no longer complains.

[emaste]

usr.sbin/makefs/mtree.c

786–787

I think I like this one slightly better

[emaste]

LGTM. I'd probably not use "avoid UAF" in the comment though, we are resetting st because it has been invalidated by freeing node->inode

[arichardson]

Update comment and use first approach