btrfs: send: remove redundant assignments to variable ret

The variable ret is being initialized to zero and also later re-assigned
to zero. In both cases the assignment is redundant since the value is
never read after the assignment and hence they can be removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>