powerpc/64e: Rewrite p4d_populate() as a static inline function

powerpc/64e: Rewrite p4d_populate() as a static inline function

Rewrite p4d_populate() as a static inline function instead of
a macro.

This change allows typechecking and would have helped detecting
a recently found bug in map_kernel_page().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1b416f8a8fe1bc3f4e01175680ce310b7eb3a1e4.1655974565.git.christophe.leroy@csgroup.eu