Skip buffer allocations if not logging

Skip buffer allocations if not logging

Currently, during IXFR we allocate a 2KB buffer for IXFR change logging
regardless of the log level. This commit introduces an early check
on the log level in dns_diff_print to avoid this.

Results in a speedup from 28% in the test case from issue #5442.

(cherry picked from commit a164492f3cfa4e6b247bc5274567dc46acb0cded)