btrfs: move up the definition of struct walk_control

In upcoming changes we need to pass struct walk_control as an argument to
replay_dir_deletes() and link_to_fixup_dir() so we need to move its
definition above the prototypes of those functions. So move it up right
below the enum that defines log replay stages and before any functions and
function prototypes are declared. Also fixup the comments while moving it
so that they comply with the preferred code style (capitalize the first
word in a sentence, end sentences with punctuation, makes lines wider and
closer to the 80 characters limit).

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