s390/mm: Fix type mismatch in get_align_mask().

s390/mm: Fix type mismatch in get_align_mask().

Commit 86f48f922ba79 ("s390/mmap: disable mmap alignment when
randomize_va_space = 0") introduced get_align_mask() with return type of
'int', while the target field 'info.align_mask' in struct
vm_unmapped_area_info is 'unsigned long'.

With currently used masks, this should not cause truncation issues, but
fix it and return 'unsigned long' to avoid future problems.

Fixes: 86f48f922ba79 ("s390/mmap: disable mmap alignment when randomize_va_space = 0")
Cc: stable@vger.kernel.org # v6.9+
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>