[PATCH] posix: Fix invalid flags test for p{write,read}v2

[Yury Khrustalev]

Two tests fail from time to time when a new flag is added for the
p{write,read}v2 functions in a new Linux kernel:

 - misc/tst-preadvwritev2
 - misc/tst-preadvwritev64v2

This disrupts when testing Glibc on a system with a newer kernel
and it seems we can try improve testing for invalid flags setting
all the bits that are not supposed to be supported (rather than
setting only the next unsupported bit).

---
Base commit: 975c8c4e22
Passes regression on aarch64 and x86 as well as build-many-glibcs
for Hurd targets. OK for trunk?

---
 misc/tst-preadvwritev2-common.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/misc/tst-preadvwritev2-common.c b/misc/tst-preadvwritev2-common.c
index 8a59ff2f9d..05fb899cd2 100644
--- a/misc/tst-preadvwritev2-common.c
+++ b/misc/tst-preadvwritev2-common.c
@@ -113,9 +113,8 @@ do_test_with_invalid_iov (void)
 static void
 do_test_with_invalid_flags (void)
 {
-  /* Set the next bit from the mask of all supported flags.  */
-  int invalid_flag = RWF_SUPPORTED != 0 ? __builtin_clz (RWF_SUPPORTED) : 2;
-  invalid_flag = 0x1 << ((sizeof (int) * CHAR_BIT) - invalid_flag);
+  /* Set all the bits that are not used by the supported flags.  */
+  int invalid_flag = ~RWF_SUPPORTED;
 
   char buf[32];
   const struct iovec vec = { .iov_base = buf, .iov_len = sizeof (buf) };
-- 
2.47.3