This patch fixes a potential for I/O to hang and LVM commands

This patch fixes a potential for I/O to hang and LVM commands
to block when a mirror under a snapshot suffers a failure.

The problem has to do with label scanning.  When a mirror suffers
a failure, the kernel blocks I/O to prevent corruption.  When
LVM attempts to repair the mirror, it scans the devices on the
system for LVM labels.  While mirrors are skipped during this
scanning process, snapshot-origins are not.  When the origin is
scanned, it kicks up I/O to the mirror (which is blocked)
underneath - causing the label scan (an thus the repair operation)
to hang.

This patch simply bypasses snapshot-origin devices when doing
labels scans (while ignore_suspended_devices() is set).  This
fixes the issue.