smb: server: Fix building with GCC 15

GCC 15 introduces -Werror=unterminated-string-initialization by default,
this results in the following build error

fs/smb/server/smb_common.c:21:35: error: initializer-string for array of 'char' is too long [-Werror=unterminated-string-ini
tialization]
   21 | static const char basechars[43] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_-!@#$%";
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

To this we are replacing char basechars[43] with a character pointer
and then using strlen to get the length.

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>