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

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

The fallback code of Linux wrapper for preadv2/pwritev2 executes
regardless of the errno code for preadv2, instead of the case where
the syscall is not supported.

This fixes it by calling the fallback code iff errno is ENOSYS. The
patch also adds tests for both invalid file descriptor and invalid
iov_len and vector count.

The only discrepancy between preadv2 and fallback code regarding
error reporting is when an invalid flags are used.  The fallback code
bails out earlier with ENOTSUP instead of EINVAL/EBADF when the syscall
is used.

Checked on x86_64-linux-gnu on a 4.4.0 and 4.15.0 kernel.

	[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.