Do 'shm_init_ns()' in an early pure_initcall

Do 'shm_init_ns()' in an early pure_initcall

This isn't really critical any more, since other patches (commit
4b077615564e: "shm: optimize exit_shm()") have caused us to not actually
need to touch the rw_mutex unless there are actual shm segments
associated with the namespace, but we really should do tne shm_init_ns()
earlier than we do now.

This, together with commit 4732e30cdb78 ("Boot up with usermodehelper
disabled") will mean that we really do initialize the initial ipc
namespace data structure before we run any tasks.

Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>