dm raid1: kcopyd should stop on error if errors handled

dm-raid1 is setting the 'DM_KCOPYD_IGNORE_ERROR' flag unconditionally
when assigning kcopyd work.  kcopyd is responsible for copying an
assigned section of disk to one or more other disks.  The
'DM_KCOPYD_IGNORE_ERROR' flag affects kcopyd in the following way:

When not set:
kcopyd will immediately stop the copy operation when an error is
encountered.

When set:
kcopyd will try to proceed regardless of errors and try to continue
copying any remaining amount.

Since dm-raid1 tracks regions of the address space that are (or
are not) in sync and it now has the ability to handle these
errors, we can safely enable this optimization.  This optimization
is conditional on whether mirror error handling has been enabled.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>