15291 zfs-tests errno flaws exposed by 15220

@@ -22,10 +22,14 @@
 /*
  * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
 
+/*
+ * Copyright 2022 MNX Cloud, Inc.
+ */
+
 #include "../file_common.h"
 
 static unsigned char bigbuffer[BIGBUFFERSIZE];
 
 /*

@@ -64,13 +68,16 @@
                 (void) printf("open %s failed %d\n", argv[1], errno);
                 exit(1);
         }
 
         do {
+                int exitcode;
+
                 if ((n = read(bigfd, &bigbuffer, bigbuffersize)) == -1) {
+                        exitcode = errno;
                         (void) printf("read failed (%ld), %d\n", n, errno);
-                        exit(errno);
+                        exit(exitcode);
                 }
 
                 for (i = 0; i < n; i++) {
                         if (bigbuffer[i] != fillchar) {
                                 (void) printf("error %s: 0x%x != 0x%x)\n",