[PATCH] Fix misreported errno on preadv2/pwritev2 (BZ#23579)

[Florian Weimer]

* Adhemerval Zanella:

> 	[BZ #23579]
> 	* misc/tst-preadvwritev2-common.c (do_test_with_invalid_fd): New
> 	test.
> 	* misc/tst-preadvwritev2.c, misc/tst-preadvwritev64v2.c (do_test):
> 	Call do_test_with_invalid_fd.
> 	* sysdeps/unix/sysv/linux/preadv2.c (preadv2): Use fallback code iff
> 	errno is ENOSYS.
> 	* sysdeps/unix/sysv/linux/preadv64v2.c (preadv64v2): Likewise.
> 	* sysdeps/unix/sysv/linux/pwritev2.c (pwritev2): Likewise.
> 	* sysdeps/unix/sysv/linux/pwritev64v2.c (pwritev64v2): Likewise.

Looks okay to me.

I see that we return ENOTSUP for invalid flags, but the kernel returns
EINVAL.  But this is a preexisting problem, unrelated to this patch.

Thanks,
Florian