btrfs-progs: init uninitialized output buf for btrfs-restore

btrfs-progs: init uninitialized output buf for btrfs-restore

A memory problem reported by valgrind as follows:
	=== Syscall param pwrite64(buf) points to uninitialised byte(s)
When running:
	# valgrind --leak-check=yes btrfs restore /dev/sda9 /mnt/backup

Because the output buf size is alloced with malloc, but the length of
output data is shorter than the sizeof(buf), so valgrind report
uninitialised byte(s).
We could use calloc to repalce malloc and clear this WARNING away.

Reported-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>