Fix coverity defects: CID 147536, 147537, 147538

coverity scan CID:147536, type: Argument cannot be negative
- may write or close fd which is negative
coverity scan CID:147537, type: Argument cannot be negative
- may call dup2 with a negative fd
coverity scan CID:147538, type: Argument cannot be negative
- may read or fchown with a negative fd

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Closes #5185

cmd/zed/zed_exec.c

@@ -106,10 +106,11 @@ _zed_exec_fork_child(uint64_t eid, const char *dir, const char *prog,
 		return;
 	} else if (pid == 0) {
 		(void) umask(022);
-		fd = open("/dev/null", O_RDWR);
+		if ((fd = open("/dev/null", O_RDWR)) != -1) {
 			(void) dup2(fd, STDIN_FILENO);
 			(void) dup2(fd, STDOUT_FILENO);
 			(void) dup2(fd, STDERR_FILENO);
+		}
 		(void) dup2(zfd, ZEVENT_FILENO);
 		zed_file_close_from(ZEVENT_FILENO + 1);
 		execle(path, prog, NULL, env);

tests/zfs-tests/cmd/dir_rd_update/dir_rd_update.c

@@ -94,6 +94,12 @@ main(int argc, char **argv)
 		int rdret;
 		int j = 0;
 
+		if (fd < 0) {
+			(void) printf("%s: open <%s> again failed:"
+			    " errno = %d\n", argv[0], dirpath, errno);
+			exit(-1);
+		}
+
 		while (j < op_num) {
 			(void) sleep(1);
 			rdret = read(fd, buf, 16);
@@ -107,6 +113,12 @@ main(int argc, char **argv)
 		int chownret;
 		int k = 0;
 
+		if (fd < 0) {
+			(void) printf("%s: open(<%s>, O_RDONLY) again failed:"
+			    " errno (decimal)=%d\n", argv[0], dirpath, errno);
+			exit(-1);
+		}
+
 		while (k < op_num) {
 			(void) sleep(1);
 			chownret = fchown(fd, 0, 0);

tests/zfs-tests/cmd/rm_lnkcnt_zero_file/rm_lnkcnt_zero_file.c

@@ -103,10 +103,15 @@ writer(void *a)
 	int ret;
 
 	while (TRUE) {
+		if (*fd != -1)
 			(void) close (*fd);
+
 		*fd = open(filebase, O_APPEND | O_RDWR | O_CREAT, 0644);
-		if (*fd < 0)
+		if (*fd == -1) {
-			perror("refreshing file");
+			perror("fail to open test file, refreshing it");
+			continue;
+		}
+
 		ret = write(*fd, "test\n", 5);
 		if (ret != 5)
 			perror("writing file");