mount-util: avoid unnecessary mount_setattr() call in make_fsmount()

mount-util: avoid unnecessary mount_setattr() call in make_fsmount()

If .attr_set is zero (and .att_clr, .propagation too), then there's no
point in calling mount_setattr().

Fixes: #37062
Note that this optimization is not precisely load-bearing anymore, since
3cc23a2c2345eb188551565349c89ec1fa8f650f got merged which removes the
only caller of make_fsmount() that might trigger it. But it's worth
fixing generic code anyway, in case it gets used like this later again.

(cherry picked from commit 249752d1ecd0f8e160d6e8633f71771b51db5a84)