sparc: mm: return true,false in kern_addr_valid()

This function's return type is bool and returns both true/false and 0/1.
This fixes the following coccicheck warning:

arch/sparc/mm/init_64.c:1652:9-10: WARNING: return of 0/1 in function
'kern_addr_valid' with return type bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>