mm: recover pud_leaf() definitions in nopmd case

This reverts one change in commit 924bd6a8 ("mm/x86: drop two
unnecessary pud_leaf() definitions").

One issue with that is it broke nopmd builds for at least both arm64 and
riscv (CONFIG_PGTABLE_LEVELS=2).  The other issue is it was overlooked that
it's a common change rather than x86 specific (relevant to the commit
message of the commit).

Normally there's no need for empty definition of pXd_leaf() because of the
fallback functions, however this logic may not apply to pgtable-nopmd.h,
because that's a header that can even be used by arch *pgtable.h headers,
which can use the *_leaf() definitions _before_ the fallback functions are
defined.  Leave it there to pass PGTABLE_LEVELS=2 builds.

Link: https://lkml.kernel.org/r/Ze8vFNV9YSdgC2S7@x1n


Fixes: 924bd6a8 ("mm/x86: drop two unnecessary pud_leaf() definitions")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403090900.OwPqmRuI-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202403101607.a42gaLOS-lkp@intel.com/


Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>