fusefs: use the fsname mount option if set

The daemon can specify fsname=XXX in its mount options. If so, the file
system should report f_mntfromname as XXX during statfs. This will show
up in the output of commands like mount and df.

Submitted by: Ali Abdallah <ali.abdallah@suse.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35090