fusefs: fix a panic on mount

Don't page fault if the file descriptor provided with "-o fd" is invalid.
This is a merge of r345419 from the projects/fuse2 branch.

Reviewed by: ngie
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19836