s390: fix alloc_pgste check in init_new_context again

git commit badb8bb9 "fix alloc_pgste check in init_new_context" fixed
the problem of 'current->mm == NULL' in init_new_context back in 2011.

git commit 3eabaee9 "KVM: s390: allow sie enablement for multi-
threaded programs" completely removed the check against alloc_pgste.

git commit 23fefe11 "s390/kvm: avoid global config of vm.alloc_pgste=1"
re-added a check against the alloc_pgste flag but without the required
check for current->mm != NULL.

For execve() called by a kernel thread init_new_context() reads from
((struct mm_struct *) NULL)->context.alloc_pgste to decide between
2K vs 4K page tables. If the bit happens to be set for the init process
it will be created with large page tables. This decision is inherited by
all the children of init, this waste quite some memory.

Re-add the check for 'current->mm != NULL'.

Fixes: 23fefe11 ("s390/kvm: avoid global config of vm.alloc_pgste=1")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>