msdosfs_rename(): handle errors from msdosfs_lookup_ino()

Properly working storage and correct filesystem structure indeed only
allow the EJUSTRETURN return code, but since the called function needs
to read directory blocks and (re)parse the content, the assert is not
neccessary hold.

PR: 276408
Reported by: John F. Carr
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D43482