mm/pkeys: Remove include of asm/mmu_context.h from pkeys.h

While trying to unify the pkey handling in show_smap() between x86 and
powerpc we stumbled across various build failures due to the order of
includes between the two arches.

Part of the problem is that linux/pkeys.h includes asm/mmu_context.h,
and the relationship between asm/mmu_context.h and asm/pkeys.h is not
consistent between the two arches.

It would be cleaner if linux/pkeys.h only included asm/pkeys.h,
creating a single integration point for the arch pkey definitions.

So this patch removes the include of asm/mmu_context.h from
linux/pkeys.h.

We can't prove that this is safe in the general case, but it passes
all the build tests I've thrown at it. Also asm/mmu_context.h is
included widely while linux/pkeys.h is not, so most likely any code
that is including linux/pkeys.h is already getting asm/mmu_context.h
from elsewhere.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>