fw_env: fix reading NVMEM device's "compatible" value

Call to fread() was changed to check for return value. The problem is it
can't be checked for returning 1 (as it is) to determine success.

We call fread() with buffer size as "size" argument. Reading any
"compatible" value shorter than buffer size will result in returning 0
even on success.

Modify code to use fstat() to determine expected read length.

This fixes regression that broke using fw_env with NVMEM devices.

Fixes: c059a22b77 ("tools: env: fw_env: Fix unused-result warning")
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

tools/env/fw_env.c

@@ -1732,6 +1732,7 @@ static int find_nvmem_device(void)
 	}
 
 	while (!nvmem && (dent = readdir(dir))) {
+		struct stat s;
 		FILE *fp;
 		size_t size;
 
@@ -1749,14 +1750,22 @@ static int find_nvmem_device(void)
 			continue;
 		}
 
-		size = fread(buf, sizeof(buf), 1, fp);
+		if (fstat(fileno(fp), &s)) {
+			fprintf(stderr, "Failed to fstat %s\n", comp);
+			goto next;
+		}
+
+		if (s.st_size >= sizeof(buf)) {
+			goto next;
+		}
+
+		size = fread(buf, s.st_size, 1, fp);
 		if (size != 1) {
 			fprintf(stderr,
 				"read failed about %s\n", comp);
-			fclose(fp);
-			return -EIO;
+			goto next;
 		}
-
+		buf[s.st_size] = '\0';
 
 		if (!strcmp(buf, "u-boot,env")) {
 			bytes = asprintf(&nvmem, "%s/%s/nvmem", path, dent->d_name);
@@ -1765,6 +1774,7 @@ static int find_nvmem_device(void)
 			}
 		}
 
+next:
 		fclose(fp);
 	}