NOMMU: support SMP dynamic percpu_alloc

The percpu code requires more functions to be implemented in the mm core
which nommu currently does not provide.  So add inline implementations
since these are largely meaningless on nommu systems.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>