selftests/core: handle missing syscall number for close_range

This improves the syscall number handling in the close_range()
selftests. This should handle any architecture.

Cc: Giuseppe Scrivano <gscrivan@redhat.com>
Cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20201218145415.801063-2-christian.brauner@ubuntu.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>