Don't set RWF_HIPRI on InvalidOffset test.

This test fails on ubuntu 18.04 because preadv2 for some reason returns
EOPNOTSUPP instead of EINVAL. Instead of root-causing the failure, I'm dropping
the flag in the preadv2 call since it isn't under test in this scenario.

PiperOrigin-RevId: 289188358

test/syscalls/linux/preadv2.cc

@@ -202,7 +202,7 @@ TEST(Preadv2Test, TestInvalidOffset) {
   iov[0].iov_len = 0;
 
   EXPECT_THAT(preadv2(fd.get(), iov.get(), /*iovcnt=*/1, /*offset=*/-8,
-                      /*flags=*/RWF_HIPRI),
+                      /*flags=*/0),
               SyscallFailsWithErrno(EINVAL));
 }