filter-sysfs: skip when device id is set

filter-sysfs: skip when device id is set

When a device id is set for a device, using an idtype other
than devname, it means that sysfs has been used with the device
to match the device id.  So, checking for a sysfs entry for the
device in filter-sysfs is redundant.  For any other cases not
covered by this (e.g. devname ids), have filter-sysfs simply
stat /sys/dev/block/major:minor to test if the device exists
in sysfs.

The extensive processing done by filter-sysfs init is removed.
It was taking an immense amount of time with many devices, e.g.
. 1024 PVs in 520 VGs
. 520 concurrent vgchange -ay <vgname> commands
. vgchange scans only PVs in the named VG (based on pvs_online
  files from a pending patch)

A large number of the vgchange commands were taking over 1 min,
and nearly half of that time was used by filter-sysfs init.
With this patch, the vgchange commands take about half the time.