x86: convert permanent_kmaps_init() from macro to inline

Impact: cleanup

This compiler warning:

  arch/x86/mm/init_32.c:515: warning: unused variable 'pgd_base'

triggers because permanent_kmaps_init() is a CPP macro in the
!CONFIG_HIGHMEM case, that does not tell the compiler that the
'pgd_base' parameter is used.

Convert permanent_kmaps_init() (and set_highmem_pages_init()) to
C inline functions - which gives the parameter a proper type and
which gets rid of the compiler warning as well.

Signed-off-by: Ingo Brueckl <ib@wupperonline.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>