btrfs: pass walk_control structure to overwrite_item()

Instead of passing the transaction and subvolume root as arguments to
overwrite_item(), pass the walk_control structure as we can grab them
from the structure. This reduces the number of arguments passed and it's
going to be needed by an incoming change that improves error reporting
for 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>