[PATCH] dm kcopyd: error accumulation fix

kcopyd should accumulate errors - otherwise I/O failures may be ignored
unintentionally.

And invert 'success' (used in a future patch), using a more intuitive
!(read_err || write_err).

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>