block: cleanup handling "start" action of the "autofs" command

block: cleanup handling "start" action of the "autofs" command

When blockd starts it uses "start" action for getting current state of
block devices. The way main_autofs() was implemented was a bit hacky
though:

1) It was calling mount_device() with TYPE_HOTPLUG
   First of all that made code harder to understand. Faking TYPE_HOTPLUG
   for a TYPE_AUTOFS made following mount_device() logic more complex.
   Secondly the only reason for that seems to be to trigger "swap" and
   extroot checks in the mount_device(). Other than that it was only
   about calling blockd_notify() anyway.

2) It wasn't consistent
   Consider a "swap" or extroot without an "autofs" set (a pretty
   expected case).
   a) During original TYPE_HOTPLUG event mount_device() would detect
      both cases early and return without calling blockd_notify().
   b) With previous main_autofs() implementation blockd_notify() was
      called for both cases.

With this change main_autofs() doesn't fake TYPE_HOTPLUG and it follows
mount_device() logic which should result in an expected & consistent
state of devices in blockd.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>