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 <firstname.lastname@example.org>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19836