[9.20] fix: usr: 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.

Backport of MR !11178

Merge branch 'backport-5442-ixfr-no-log-alloc-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11192