btrfs: pass walk_control structure to drop_one_dir_item() and helpers

Instead of passing the transaction as an argument to drop_one_dir_item()
and its helpers (link_to_fixup_dir() and unlink_inode_for_log_replay()),
pass the walk_control structure as we can access the transaction from it
and the subvolume root. This is going to be needed by an incoming change
that improves error reporting for log replay and also reduces the number
of arguments passed to link_to_fixup_dir().

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