[22841] Improve `max_allocations` calculation on SHM transport #1772

Job	Run time
sanitizers-ci / asan_fastdds_build	7m 7s
sanitizers-ci / tsan_fastdds_test	3h 57m 30s
sanitizers-ci / asan_fastdds_test	5h 0m 44s
sanitizers-ci / asan_discovery_server_test	7m 21s
	9h 12m 42s

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