fs: dlm: Simplify buffer size computation in dlm_create_debug_file()

Use sizeof(name) instead of the equivalent, but hard coded,
DLM_LOCKSPACE_LEN + 8.

This is less verbose and more future proof.

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>