fs: dlm: Remove some useless memset()

There is no need to clear the buffer used to build the file name.

snprintf() already guarantees that it is NULL terminated and such a
(useless) precaution was not done for the first string (i.e
ls_debug_rsb_dentry)

So, save a few LoC.

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>