Protect calls to GeoIP_region_by_addr with our mutex

Tracing through the call stack we have:
GeoIP_region_by_addr
which calls _get_region
which calls GeoIP_assign_region_by_inetaddr
which calls_GeoIP_seek_record
which clobbers our global netmask.
Expand comment for the mutex
GeoIP_record_by_addr also mutates netmask
Merge pull request #9 from dgryski/mutex-for-region-by-addr

Protect calls to GeoIP calls with our mutex