s390/mm: split arch/s390/mm/pgtable.c

The pgtable.c file is quite big, before it grows any larger split it
into pgtable.c, pgalloc.c and gmap.c. In addition move the gmap related
header definitions into the new gmap.h header and all of the pgste
helpers from pgtable.h to pgtable.c.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/asm-offsets.c

@@ -12,6 +12,7 @@
 #include <asm/idle.h>
 #include <asm/vdso.h>
 #include <asm/pgtable.h>
+#include <asm/gmap.h>
 
 /*
  * Make sure that the compiler is new enough. We want a compiler that