dm: simplify start of block stats accounting for bio-based

No apparent need to generic_start_io_acct() until before the IO is ready
for submission.  start_io_acct() is the proper place to do this
accounting -- it is also where DM accounts for pending IO and, if
enabled, starts dm-stats accounting.

Replace start_io_acct()'s part_round_stats() with generic_start_io_acct().
This eliminates needing to take part_stat_lock() multiple times when
starting an IO on bio-based devices.

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