btrfs: add tracking of read blocks for read policy

btrfs: add tracking of read blocks for read policy

Track number of read blocks in the whole filesystem. The counter is
initialized when devices are opened. The counter is increased at
btrfs_submit_dev_bio() if the stats tracking is enabled (depends on the
read policy).  Stats tracking is disabled by default and is enabled
through fs_devices::collect_fs_stats when required.

The code is not under the EXPERIMENTAL define, as stats can be expanded
to include write counts and other performance counters, with the user
interface independent of its internal use.

This is an in-memory-only feature, not related to the dev error stats.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>