powerpc: mman.h export fixups

Commit ef3d3246 ("powerpc/mm: Add Strong
Access Ordering support") in the powerpc/{next,master} tree caused the
following in a powerpc allmodconfig build:

usr/include/asm/mman.h requires linux/mm.h, which does not exist in exported headers

We should not use CONFIG_PPC64 in an unprotected (by __KERNEL__)
section of an exported include file and linux/mm.h is not exported.  So
protect the whole section that is CONFIG_PPC64 with __KERNEL__ and put
the two introduced includes in there as well.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>