ceph: don't return a value from void function

This fixes a build warning to that effect.

Fixes: 1a829ff2

 ("ceph: no need to check return value of debugfs_create functions")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>