Simplify sendfile.c test program for use on an NFS partition.

Simplify sendfile.c test program for use on an NFS partition.

* testsuite/systemtap.syscall/sendfile.c: Simplify testcase. Originally
  when compiled 32-bits on a 64-bit system and run on an NFS partition,
  fstat() returns an invalid size of the newly created file. This invalid
  size was verified with strace. Since we know the size of the file
  anyway, just use it directly, which avoids the NFS problem.