mount: add mount with ignore=1 for unsupported filesystems

mount: add mount with ignore=1 for unsupported filesystems

So far a check for unsupported filesystem was in the mount_add_list
which was simply stopping mount from being added to the global list.

This resulted in mount_dev_add continuously not being able to find a
mount for the given block device and trying to add it over and over.
That was non-optimal becuse with unsupported filesystem present the
code was checking all its parameters every second.

Fix this by:
1) Moving check out of the mount_add_list to keep all logic in the
   caller function.
2) Adding mount with ignore=1 for unsupported filesystem instead of
   ignoring it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>