btrfs: don't report readahead errors and don't update statistics

As readahead is an optimization, all errors are usually filtered out,
but still properly handled when the real read call is done. The commit
5e9d3982 ("btrfs: readpages() should submit IO as read-ahead") added
REQ_RAHEAD to readpages() because that's only used for readahead
(despite what one would expect from the callback name).

This causes a flood of messages and inflated read error stats, so skip
reporting in case it's readahead.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=202403


Reported-by: LimeTech <tomm@lime-technology.com>
Fixes: 5e9d3982 ("btrfs: readpages() should submit IO as read-ahead")
CC: stable@vger.kernel.org # 4.19+
Signed-off-by: David Sterba <dsterba@suse.com>