fs: dlm: Fix the size of a buffer in dlm_create_debug_file()

8 is not the maximum size of the suffix used when creating debugfs files.

Let the compiler compute the correct size, and only give a hint about the
longest possible string that is used.

When building with W=1, this fixes the following warnings:

  fs/dlm/debug_fs.c: In function ‘dlm_create_debug_file’:
  fs/dlm/debug_fs.c:1020:58: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
   1020 |         snprintf(name, DLM_LOCKSPACE_LEN + 8, "%s_waiters", ls->ls_name);
        |                                                          ^
  fs/dlm/debug_fs.c:1020:9: note: ‘snprintf’ output between 9 and 73 bytes into a destination of size 72
   1020 |         snprintf(name, DLM_LOCKSPACE_LEN + 8, "%s_waiters", ls->ls_name);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  fs/dlm/debug_fs.c:1031:50: error: ‘_queued_asts’ directive output may be truncated writing 12 bytes into a region of size between 8 and 72 [-Werror=format-truncation=]
   1031 |         snprintf(name, DLM_LOCKSPACE_LEN + 8, "%s_queued_asts", ls->ls_name);
        |                                                  ^~~~~~~~~~~~
  fs/dlm/debug_fs.c:1031:9: note: ‘snprintf’ output between 13 and 77 bytes into a destination of size 72
   1031 |         snprintf(name, DLM_LOCKSPACE_LEN + 8, "%s_queued_asts", ls->ls_name);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 541adb0d ("fs: dlm: debugfs for queued callbacks")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>