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

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);