s390/mm: use generic mm_hooks

With git commit 3446c13b
"s390/mm: four page table levels vs. fork"
s390 dropped its architecture specific version of arch_dup_mmap.

Now all functions defined by include/asm-generic/mm_hooks.h are
identical to the s390 versions. Use the generic header.

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