mm/pkeys, x86, powerpc: Display pkey in smaps if arch supports pkeys

Currently the architecture specific code is expected to display the
protection keys in smap for a given vma. This can lead to redundant
code and possibly to divergent formats in which the key gets
displayed.

This patch changes the implementation. It displays the pkey only if
the architecture support pkeys, i.e arch_pkeys_enabled() returns true.

x86 arch_show_smap() function is not needed anymore, delete it.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
[mpe: Split out from larger patch, rebased on header changes]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>