msdosfs: fix vnode leak with msdosfs_rename()

This could happen when failing due to disappearing source file.

Reviewed By: kib
Tested by: pho
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27338

(cherry picked from commit b8073b3c74342113398f36a1c68f442ae9e092e6)