dm: eliminate copying of dm_io fields in dm_io_dec_pending

There is no need for dm_io_dec_pending() to copy dm_io fields
anymore now that DM provides its own pending_io counters again.

The race documented in commit d208b894 ("dm: fix mempool NULL
pointer race when completing IO") no longer exists now that block
core's in_flight counters aren't used to signal all dm_io is
complete.

Also, rename {start,end}_io_acct to dm_{start,end}_io_acct.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>