When writing to a mirror, the log must be updated first.  Failure
to update the log could result in the log not properly reflecting
the state of the mirror if the machine should crash.

We change the return type of the rh_flush function to give us
the ability to check if a log write was successful.  If the
log write was unsuccessful, we fail the writes to avoid the
case where the log does not properly reflect the state of the
mirror.

A follow-up patch - which is dependent on the ability to
requeue I/O's to core device-mapper - will requeue the I/O's
for retry (allowing the mirror to be reconfigured.)
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>