lsfd: use memset explicitly to fill bpf_attr with zero

lsfd: use memset explicitly to fill bpf_attr with zero

The original code used initialize-lists for initializing
variables (attr) type of union bpf_attr.

However, the folloing syscalls failed with EINVAL:

     syscall(SYS_bpf, BPF_PROG_GET_FD_BY_ID, &attr, sizeof(attr));
     syscall(SYS_bpf, BPF_MAP_GET_FD_BY_ID, &attr, sizeof(attr));

CHECK_ATTR macro of Linux might cause EINVAL:

    /* helper macro to check that unused fields 'union bpf_attr' are zero */
    #define CHECK_ATTR(CMD) \
	    memchr_inv((void *) &attr->CMD##_LAST_FIELD + \
		       sizeof(attr->CMD##_LAST_FIELD), 0, \
		       sizeof(*attr) - \
		       offsetof(union bpf_attr, CMD##_LAST_FIELD) - \
		       sizeof(attr->CMD##_LAST_FIELD)) != NULL

I doubt initialize-lists worked well.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>