Relatively deterministic panic with sendfile(2) when running tests in the sxlock code

[Enji Cooper (yaneurabeya)]

Hi,
	I’m seeing a semi-deterministic panic on 12.0-ALPHA9 related to sendfile(2) when running sendfile_test on the host: https://pastebin.com/raw/6Y7xg0ki; it looks like it’s crashing in the sxlock code when calling sblock on a sockbuf. Are there any commands in gdb you would like me to run to display lock state?
	Repro:

mkdir /path/to/git/checkout
cd /path/to/git/checkout
git clone https://github.com/ngie-eign/freebsd/tree/sendfile_tests .
git checkout sendfile_tests
(cd lib/libc/tests/sys/; make obj; make; sudo make install)
kyua test -k /usr/tests/lib/libc/sys/Kyuafile sendfile_test

Thank you!
-Enji
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