new panic in cpu_reset() with WITNESS

  New panic has been introduced somewhere between
r229851 and r229932, that happens on shutdown if
kernel has WITNESS and doesn't have WITNESS_SKIPSPIN.

Uptime: 1h0m17s
Rebooting...
panic: mtx_lock_spin: recursed on non-recursive mutex cnputs_mtx _at_ /usr/src/head/sys/kern/kern_cons.c:500
cpuid = 0
KDB: enter: panic
[ thread pid 1 tid 100001 ]
Stopped at      kdb_enter+0x3b: movq    $0,0x514d32(%rip)
db>
db> bt
Tracing pid 1 tid 100001 td 0xfffffe0001d5e000
kdb_enter() at kdb_enter+0x3b
panic() at panic+0x1c7
_mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x10f
cnputs() at cnputs+0x7a
putchar() at putchar+0x11f
kvprintf() at kvprintf+0x83
vprintf() at vprintf+0x85
printf() at printf+0x67
witness_checkorder() at witness_checkorder+0x773
_mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x99
uart_cnputc() at uart_cnputc+0x3e
cnputc() at cnputc+0x4c
cnputs() at cnputs+0x26
putchar() at putchar+0x11f
kvprintf() at kvprintf+0x83
vprintf() at vprintf+0x85
printf() at printf+0x67
cpu_reset() at cpu_reset+0x81
kern_reboot() at kern_reboot+0x3a5
--More--^M        ^Msys_reboot() at sys_reboot+0x42
amd64_syscall() at amd64_syscall+0x39e
Xfast_syscall() at Xfast_syscall+0xf7
--- syscall (55, FreeBSD ELF64, sys_reboot), rip = 0x40ea3c, rsp = 0x7fffffffd6d8, rbp = 0x49 ---
db>
db> show locks
exclusive sleep mutex Giant (Giant) r = 0 (0xffffffff809bc560) locked _at_ /usr/src/head/sys/kern/kern_module.c:101
exclusive spin mutex smp rendezvous (smp rendezvous) r = 0 (0xffffffff80a08840) locked _at_ /usr/src/head/sys/kern/kern_shutdown.c:542
db>

So the problem is that we are holding smp rendezvous mutex during the cpu_reset().
No mutexes should be obtained after it. However, since cpu_reset() does priting
we obtain cnputs_mtx, and later obtain uart_hwmtx. The latter is hardcoded in
the subr_witness.c as mutex to obtain before smp rendezvous, this triggers
yet another printf from witness, that finally panics due to recursing on
cnputs_mtx.

-- 
Totus tuus, Glebius.