fix dma-buf/udmabuf selftest

This patch fixes the udmabuf selftest. Currently the selftest is broken.
I fixed the selftest by setting the F_SEAL_SHRINK seal on the memfd
file descriptor which is required by udmabuf and added the test to
the selftest Makefile.

Signed-off-by: Tom Murphy <murphyt7@tcd.ie>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>