fs/qnx6: Fix building with GCC 15

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"};
      |                                                  ^~~~~~

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
Link: https://lore.kernel.org/r/20241004195132.1393968-1-brahmajit.xyz@gmail.com
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <brauner@kernel.org>