modpost: avoid false-positive file open error

One problem of grab_file() is that it cannot distinguish the following
two cases:

 - It cannot read the file (the file does not exist, or read permission
   is not set)

 - It can read the file, but the file size is zero

This is because grab_file() calls mmap(), which requires the mapped
length is greater than 0. Hence, grab_file() fails for both cases.

If an empty header file were included for checksum calculation, the
following warning would be printed:

  WARNING: modpost: could not open ...: Invalid argument

An empty file is a valid source file, so it should not fail.

Use read_text_file() instead. It can read a zero-length file.
Then, parse_file() will succeed with doing nothing.

Going forward, the first case (it cannot read the file) is a fatal
error. If the source file from which an object was compiled is missing,
something went wrong.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/sumversion.c

@@ -258,9 +258,8 @@ static int parse_file(const char *fname, struct md4_ctx *md)
 	char *file;
 	unsigned long i, len;
 
-	file = grab_file(fname, &len);
-	if (!file)
-		return 0;
+	file = read_text_file(fname);
+	len = strlen(file);
 
 	for (i = 0; i < len; i++) {
 		/* Collapse and ignore \ and CR. */
@@ -287,7 +286,7 @@ static int parse_file(const char *fname, struct md4_ctx *md)
 
 		add_char(file[i], md);
 	}
-	release_file(file, len);
+	free(file);
 	return 1;
 }
 /* Check whether the file is a static library or not */