iotests/108: Fix when missing user_allow_other

iotests/108: Fix when missing user_allow_other

FUSE exports' allow-other option defaults to "auto", which means that it
will try passing allow_other as a mount option, and fall back to not
using it when an error occurs.  We make no effort to hide fusermount's
error message (because it would be difficult, and because users might
want to know about the fallback occurring), and so when allow_other does
not work (primarily when /etc/fuse.conf does not contain
user_allow_other), this error message will appear and break the
reference output.

We do not need allow_other here, though, so we can just pass
allow-other=off to fix that.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220421142435.569600-1-hreitz@redhat.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>