(build 1 - initial: kprintf missing 3rd arg -> -Werror=format= stopped build)
msk_hevent_replica.c:213:49: error: format '%d' expects a matching 'int' argument [-Werror=format=]
cc1: all warnings being treated as errors
(fixed: added put_idx argument)

(build 2 - FINAL, clean)
$ cd /root/kmod3807 && tar xzf /root/msk3807.tgz && make
cc -O2 -pipe -D_KERNEL ... -std=gnu11 -Werror -DKLD_MODULE -nostdinc ... -c msk_hevent_replica.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o msk_hevent_replica.ko msk_hevent_replica.o
(no errors, no warnings)
-rw-r--r-- 1 root wheel 9424 msk_hevent_replica.ko