btrfs: hooks for readahead

This adds the hooks needed for readahead. In the readpage_end_io_hook,
the extent state is checked for the EXTENT_READAHEAD flag. Only in this
case the readahead hook is called, to keep the impact on non-ra as low
as possible.
Additionally, a hook for a failed IO is added, otherwise readahead would
wait indefinitely for the extent to finish.

Changes for v2:
 - eliminate race condition

Signed-off-by: Arne Jansen <sensille@gmx.net>