Clearing the DE_RENAME flag, done near the end of msdosfs_rename(),
can result in overwriting freed memory: since fvp is not locked
at that point, ip might no longer be valid.
Instead of trying to invent a safe way of resetting the flag
(https://reviews.freebsd.org/D27337), drop it altogether.
The cases it's supposed to prevent from shouldn't break anything.
Note that this is relatively untested, as msdosfs_rename() is already
broken in a different way; see https://bugs.freebsd.org/257522.
I'm still interested in feedback, in particular about the approach.