btrfs: add current log leaf, key and slot to struct walk_control

A lot of the log replay functions get passed the current log leaf being
processed as well as the current slot and the key at that slot. Instead
of passing them as parameters, add them to struct walk_control so that
we reduce the numbers of parameters. This is also going to be needed to
further changes that improve error reporting during log replay.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana