btrfs-progs: use calloc instead of malloc+memset

btrfs-progs: use calloc instead of malloc+memset

This patch is generated from a coccinelle semantic patch:

	identifier t;
	expression e;
	statement s;
	@@
	-t = malloc(e);
	+t = calloc(1, e);
	(
	if (!t) s
	|
	if (t == NULL) s
	|
	)
	-memset(t, 0, e);

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
[squashed patches into one]
Signed-off-by: David Sterba <dsterba@suse.com>