dlm: replace one-element array with fixed size array

One-element arrays are deprecated. So, replace one-element array with
fixed size array member in struct dlm_ls, and refactor the rest of the
code, accordingly.

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/228
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836
Link: https://lore.kernel.org/lkml/Y0W5jkiXUkpNl4ap@mail.google.com/



Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Teigland <teigland@redhat.com>