mm: bring back /sys/kernel/mm

Commit da29bd36 ("mm/mm_init.c: make creation of the mm_kobj happen
earlier than device_initcall") changed to pure_initcall(mm_sysfs_init).

That's too early: mm_sysfs_init() depends on core_initcall(ksysfs_init)
to have made the kernel_kobj directory "kernel" in which to create "mm".

Make it postcore_initcall(mm_sysfs_init).  We could use core_initcall(),
and depend upon Makefile link order kernel/ mm/ fs/ ipc/ security/ ...
as core_initcall(debugfs_init) and core_initcall(securityfs_init) do;
but better not.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>