nbd/trivial: fix type cast for ioctl

This fixes ioctl behavior on powerpc e6500 platforms with 64bit kernel and 32bit
userspace. The current type cast has no effect there and the value passed to the
kernel is still 0. Probably an issue related to the compiler, since I'm assuming
the same configuration works on a similar setup on x86.

Also ensure consistency with previous type cast in TRACE message.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Message-Id: <1428058914-32050-1-git-send-email-bogdan.purcareata@freescale.com>
Cc: qemu-stable@nongnu.org
[Fix parens as noticed by Michael. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

(cherry picked from commit d064d9f3)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>