FreeBSD: Ensure that zfs_getattr() initializes the va_rdev field

Otherwise the field is left uninitialized, leading to a possible kernel
memory disclosure to userspace or to the network. Use the same
initialization value we use in zfsctl_common_getattr().

Reported-by: KMSAN
Sponsored-by: The FreeBSD Foundation
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ed Maste <emaste@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #15639