Revert "dm mirror: use all available legs on multiple failures"

This reverts commit 12a7cf5b.

This commit apparently attempted to fix an issue that didn't really
exist, furthermore: this commit is the source of deadlocks and crashes
seen in multiple cases related to failing the primary mirror dev while
syncing.

Reported-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>