uapi: change the type of struct statx_timestamp.tv_nsec to unsigned

uapi: change the type of struct statx_timestamp.tv_nsec to unsigned

The comment asserting that the value of struct statx_timestamp.tv_nsec
must be negative when statx_timestamp.tv_sec is negative, is wrong, as
could be seen from the following example:

	#define _FILE_OFFSET_BITS 64
	#include <assert.h>
	#include <fcntl.h>
	#include <stdio.h>
	#include <sys/stat.h>
	#include <unistd.h>
	#include <asm/unistd.h>
	#include <linux/stat.h>

	int main(void)
	{
		static const struct timespec ts[2] = {
			{ .tv_nsec = UTIME_OMIT },
			{ .tv_sec = -2, .tv_nsec = 42 }
		};
		assert(utimensat(AT_FDCWD, ".", ts, 0) == 0);

		struct stat st;
		assert(stat(".", &st) == 0);
		printf("st_mtim.tv_sec = %lld, st_mtim.tv_nsec = %lu\n",
		       (long long) st.st_mtim.tv_sec,
		       (unsigned long) st.st_mtim.tv_nsec);

		struct statx stx;
		assert(syscall(__NR_statx, AT_FDCWD, ".", 0, 0, &stx) == 0);
		printf("stx_mtime.tv_sec = %lld, stx_mtime.tv_nsec = %lu\n",
		       (long long) stx.stx_mtime.tv_sec,
		       (unsigned long) stx.stx_mtime.tv_nsec);

		return 0;
	}

It expectedly prints:
st_mtim.tv_sec = -2, st_mtim.tv_nsec = 42
stx_mtime.tv_sec = -2, stx_mtime.tv_nsec = 42

The more generic comment asserting that the value of struct
statx_timestamp.tv_nsec might be negative is confusing to say the least.

It contradicts both the struct stat.st_[acm]time_nsec tradition and
struct timespec.tv_nsec requirements in utimensat syscall.
If statx syscall ever returns a stx_[acm]time containing a negative
tv_nsec that cannot be passed unmodified to utimensat syscall,
it will cause an immense confusion.

Fix this source of confusion by changing the type of struct
statx_timestamp.tv_nsec from __s32 to __u32.

Fixes: 0852e42038d4 ("statx: Add a system call to make enhanced file info available")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
cc: mtk.manpages@gmail.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>