gfs2: constify rhashtable_params

rhashtable_params are not supposed to change at runtime. All
Functions rhashtable_* working with const rhashtable_params
provided by <linux/rhashtable.h>. So mark the non-const structs
as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>