memfd_test: Make it work on 32-bit systems

This test currently fails on 32-bit systems since we use u64 type to pass the
flags to fcntl.

This commit changes this to use 'unsigned int' type for flags to fcntl making it
work on 32-bit systems.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>