export/fuse: Pass default_permissions for mount

We do not do any permission checks in fuse_open(), so let the kernel do
them.  We already let fuse_getattr() report the proper UNIX permissions,
so this should work the way we want.

This causes a change in 308's reference output, because now opening a
non-writable export with O_RDWR fails already, instead of only actually
attempting to write to it.  (That is an improvement.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210625142317.271673-2-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>