btrfs: tracepoints: simplify raid56 events

After commit 6bfd0133 ("btrfs: raid56: switch scrub path to use a
single function"), the raid56 implementation no longer uses different
endio functions for RMW/recover/scrub.

All read operations end in submit_read_wait_bio_list(), while all write
operations end in submit_write_bios().  This means quite some trace
events are out-of-date and no longer utilized.

This patch would unify the trace events into just two:

- trace_raid56_read()
  Replaces trace_raid56_read_partial(), trace_raid56_scrub_read() and
  trace_raid56_scrub_read_recover().

- trace_raid56_write()
  Replaces trace_raid56_write_stripe() and
  trace_raid56_scrub_write_stripe().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>