Remove unneeded code from domain migration.

This patch removes unneeded code from be_migrate_thread_to_domain().
This function no longer needs to check the number of online
CPUs since the CPU mask for a given domain is now returned directly
from read_mapping().
Cosmetic: Fix error string to match fn name.

Patch cleans up an error message to match the
migrate-to-domain API.
read_mapping() breaks on NR_CPUS not div by 32

This patch fixes a bug in read_mapping(), which is used for
reading CPU and cluster (domain) mappings. read_mapping()
did not account for the case when NR_CPUS is not evenly
divisible by 32 (quite common when NR_CPUS < 32).