msdosfs: fix double unlock if the source file disappears

We would unlock fvp here, only to unlock it again below,
just before "bad".

Reviewed By: kib
Tested by: pho
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27339

(cherry picked from commit cb696212495995c8374d9fe12fbae340614af45c)