kobject: Export kobj_ns_grab_current() and kobj_ns_drop()

Make it possible to call these two functions from a kernel module.
Note: despite their name, these two functions can be used meaningfully
independent of kobjects. A later patch will add calls to these
functions from the SRP driver because this patch series modifies the
SRP driver such that it can hold a reference to a namespace that can
last longer than the lifetime of the process through which the
namespace reference was obtained.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>