Speedup DTLS max fragment size calculation for MTU limits #6135

Job	Run time
build library (--disable-asm)	32s
build library (--disable-asm --enable-all-crypto --enable-mlkem --enable-lms --enable-xmss --enab...	54s
build library (--enable-intelasm --enable-sp-asm --enable-mlkem --enable-lms --enable-xmss --enab...	33s
build library (--enable-intelasm --enable-sp-asm --enable-all-crypto --enable-mlkem --enable-lms ...	57s
build library (--enable-intelasm --enable-sp-asm)	29s
build library (--disable-asm --enable-mlkem --enable-lms --enable-xmss --enable-mldsa)	34s
	3m 59s

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