fs/qnx6: Fix building with GCC 15

qnx6_checkroot() had been using weirdly spelled initializer - it needed
to initialize 3-element arrays of char and it used NUL-padded
3-character string literals (i.e. 4-element initializers, with
completely pointless zeroes at the end).

That had been spotted by gcc-15[*]; prior to that gcc quietly dropped
the 4th element of initializers.

However, none of that had been needed in the first place - all this
array is used for is checking that the first directory entry in root
directory is "." and the second - "..".  The check had been expressed as
a loop, using that match_root[] array.  Since there is no chance that we
ever want to extend that list of entries, the entire thing is much too
fancy for its own good; what we need is just a couple of explicit
memcmp() and that's it.

[*]: fs/qnx6/inode.c: In function ‘qnx6_checkroot’:
fs/qnx6/inode.c:182:41: error: initializer-string for array of ‘char’ is too long [-Werror=unterminated-string-initialization]
  182 |         static char match_root[2][3] = {".\0\0", "..\0"};
      |                                         ^~~~~~~
fs/qnx6/inode.c:182:50: error: initializer-string for array of ‘char’ is too long [-Werror=unterminated-string-initialization]
  182 |         static char match_root[2][3] = {".\0\0", "..\0"};
      |                                                  ^~~~~~

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/qnx6/inode.c

@@ -179,8 +179,7 @@ static int qnx6_statfs(struct dentry *dentry, struct kstatfs *buf)
  */
 static const char *qnx6_checkroot(struct super_block *s)
 {
-	static char match_root[2][3] = {".\0\0", "..\0"};
-	int i, error = 0;
+	int error = 0;
 	struct qnx6_dir_entry *dir_entry;
 	struct inode *root = d_inode(s->s_root);
 	struct address_space *mapping = root->i_mapping;
@@ -189,11 +188,9 @@ static const char *qnx6_checkroot(struct super_block *s)
 	if (IS_ERR(folio))
 		return "error reading root directory";
 	dir_entry = kmap_local_folio(folio, 0);
-	for (i = 0; i < 2; i++) {
-		/* maximum 3 bytes - due to match_root limitation */
-		if (strncmp(dir_entry[i].de_fname, match_root[i], 3))
-			error = 1;
-	}
+	if (memcmp(dir_entry[0].de_fname, ".", 2) ||
+	    memcmp(dir_entry[1].de_fname, "..", 3))
+		error = 1;
 	folio_release_kmap(folio, dir_entry);
 	if (error)
 		return "error reading root directory.";