btrfs: rename replay_dest member of struct walk_control to root

btrfs: rename replay_dest member of struct walk_control to root

Everywhere else we refer to a subvolume root we are replaying to simply
as 'root', so rename from 'replay_dest' to 'root' for consistency and
having a more meaningful and shorter name. While at it also update the
comment to be more detailed and comply to preferred style (first word in
a sentence is capitalized and sentence ends with punctuation).

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