filter-mpath: detect partitions of mpath components

filter-mpath: detect partitions of mpath components

We use mpath filtering (enabled by devices/multipath_component_detection=1
lvm.conf setting) to avoid a situation in which we could end up with
duplicate PVs found. We need to filter out the mpath components and
use only the top-level multipath mapping instead for PV scans.

However, if the there are partitions on multipath components, we need
to filter out these partitions. This patch fixes it so those
partitions found on multipath components are filtered as well.

For example, let's consider following configuration:
The sda and sdb are mpath components, sda1 and sdb1 the partitions
on these components, mpath-test the mpath mapping and mpath-test1
the partition mapping - created automatically by kpartx right
after mpath-test creation. The PV resides on top.

       (LVM PV)
          |
      mpath-test1
          |
      mpath-test
          |
sda1 ---------- sdb1
   \ |        |/
    sda      sdb

E.g. for sda1 and sdb1, the code will detect this and it skips
the partition that belongs to the multipath component:
  <snippet from the log>
    #filters/filter-mpath.c:156         /dev/sda1: Device is a partition, using primary device /dev/sda for mpath component detection
    130 #ioctl/libdm-iface.c:1724         dm status   (253:2) OF[16384](*1)
    131 #filters/filter-mpath.c:196         /dev/sda1: Skipping mpath component device
  </snippet from the log>

Othewise, we'd see the same PV label on sda1/sdb1 and mpath-test1
at the same time ending up with "Duplicate PV found...".