orangefs: Constify struct kobj_type

'struct kobj_type' is not modified. It is only used in kobject_init()
which takes a 'const struct kobj_type *ktype' parameter.

Constifying this structure moves some data to a read-only section,
so increase over all security.

On a x86_64, compiled with defconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   7036	   2136	     56	   9228	   240c	fs/orangefs/orangefs-sysfs.o

After:
======
   text	   data	    bss	    dec	    hex	filename
   7484	   1880	     56	   9420	   24cc	fs/orangefs/orangefs-sysfs.o

Signed-off-by: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>