files: Don't perform fd rst stages, if they will be no-op

The recv-fd stage is only required if we created any transport fd.
The post-open one is only required if there's at least one fdtype
with post-open callback in ops.

Both things can be evaluated in advance and unneeded stages can
be skipped.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>