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

[Yury Khrustalev]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=58a31b4316f1f687184eb147ffa1c676bc6a190e

commit 58a31b4316f1f687184eb147ffa1c676bc6a190e
Author: Yury Khrustalev <yury.khrustalev@arm.com>
Date:   Thu Nov 6 12:57:58 2025 +0000

    posix: Fix invalid flags test for p{write,read}v2
    
    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).
    
    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Diff:
---
 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) };