configure: Update configure so that open_by_handle_at check returns correct value

According to David Gibson for some compiler/libc combinations, open_by_handle_at
test in configure isn't quite right: because the file_handle pointer is never
dereferenced, gcc doesn't complain even if it is undefined. Change the test
as suggested by him.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
