Fix regression in do_write(s) causing significant performance issues when using large (>10meg) writes #949

Job	Run time
openssl-1.0.2u	2m 28s
openssl-1.1.0l	1m 33s
ruby-versions / Generate Ruby versions	3s
openssl-1.1.1w	1m 39s
openssl-3.0.12	2m 46s
openssl-3.1.4	2m 52s
openssl-3.2.0	3m 27s
libressl-3.1.5	2m 3s
libressl-3.2.7	2m 11s
libressl-3.3.6	2m 14s
libressl-3.4.3	2m 11s
libressl-3.5.3	2m 19s
libressl-3.6.3	2m 15s
libressl-3.7.3	2m 22s
libressl-3.8.1	2m 19s
openssl-3.0.12 fips	2m 41s
openssl-3.1.4 fips	2m 46s
openssl-3.2.0 fips	3m 20s
openssl-head	3m 31s
openssl-head fips	3m 13s
ubuntu-22.04 2.7	1m 0s
ubuntu-22.04 3.0	52s
ubuntu-22.04 3.1	55s
ubuntu-22.04 3.2	59s
ubuntu-22.04 3.3	52s
ubuntu-22.04 head	55s
ubuntu-22.04 truffleruby	1m 32s
ubuntu-22.04 truffleruby-head	1m 36s
ubuntu-20.04 2.7	54s
ubuntu-20.04 3.0	54s
ubuntu-20.04 3.1	51s
ubuntu-20.04 3.2	52s
ubuntu-20.04 3.3	49s
ubuntu-20.04 head	50s
ubuntu-20.04 truffleruby-head	1m 26s
macos-latest 2.7	1m 25s
macos-latest 3.0	51s
macos-latest 3.1	1m 22s
macos-latest 3.2	57s
macos-latest 3.3	1m 7s
macos-latest head	2m 58s
macos-latest truffleruby-head	1m 52s
windows-latest 2.7	5m 24s
windows-latest 3.0	4m 44s
windows-latest 3.1	4m 1s
windows-latest 3.2	5m 9s
windows-latest 3.3	5m 12s
windows-latest ucrt	4m 25s
windows-latest mswin	3m 44s
	1h 46m 41s

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