s390/mm: fix per vma lock fault handling

With per-vma locks, handle_mm_fault() may return non-fatal error
flags. In this case the code should reset the fault flags before
returning.

Fixes: e06f47a1 ("s390/mm: try VMA lock-based page fault handling first")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>