fs: add fcntl() interface for setting/getting write life time hints

fs: add fcntl() interface for setting/getting write life time hints

We have a pwritev2(2) interface based on passing in flags. Add an
fcntl interface for querying these flags, and also for setting them
as well:

F_GET_RW_HINT		Returns the read/write hint set. Right now it
			will be one of the WRITE_LIFE_* values.

F_SET_RW_HINT		Pass in rw_hint type to set the read/write hint.
			Only WRITE_LIFE_* hints are currently supported.
			Returns 0 on succes, -1 otherwise.

Sample program testing/implementing basic setting/getting of write
hints is below.

/*
 * writehint.c: check or set a file/inode write hint
 */

static char *str[] = { "WRITE_LIFE_NONE", "WRITE_LIFE_SHORT",
			"WRITE_LIFE_MEDIUM", "WRITE_LIFE_LONG",
			"WRITE_LIFE_EXTREME" };

int main(int argc, char *argv[])
{
	int hint = -1, fd, ret;

	if (argc < 2) {
		fprintf(stderr, "%s: dev <hint>\n", argv[0]);
		return 1;
	}

	fd = open(argv[1], O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 2;
	}

	if (argc > 2)
		hint = atoi(argv[2]);

	if (hint == -1) {
		ret = fcntl(fd, F_GET_RW_HINT);
		if (ret < 0) {
			perror("fcntl: F_GET_RW_HINT");
			return 3;
		}
		hint = ret;
	} else {
		ret = fcntl(fd, F_SET_RW_HINT, hint);
		if (ret < 0) {
			perror("fcntl: F_SET_RW_HINT");
			return 4;
		}
	}

	printf("%s: %shint %s\n", argv[1], hint != -1 ? "set " : "", str[hint]);
	close(fd);
	return 0;
}

Signed-off-by: Jens Axboe <axboe@kernel.dk>