cam iosched: Add a handler for the quanta sysctl to enforce valid values

[fk_fabiankeil.de]

Summary

cam iosched: Add a handler for the quanta sysctl to enforce valid values

Invalid values can result in devision-by-zero panics or other
undefined behaviour so lets not allow them. Example panic:

Fatal trap 18: integer divide fault while in kernel mode
cpuid = 0; apic id = 00
instruction pointer     = 0x20:0xffffffff802998e5
stack pointer           = 0x28:0xfffffe01ea3bf8d0
frame pointer           = 0x28:0xfffffe01ea3bf900
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 12 (swi4: clock (0))
trap number             = 18
panic: integer divide fault
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe01ea3bf510
vpanic() at vpanic+0x186/frame 0xfffffe01ea3bf590
panic() at panic+0x43/frame 0xfffffe01ea3bf5f0
trap_fatal() at trap_fatal+0x322/frame 0xfffffe01ea3bf640
trap() at trap+0x5e/frame 0xfffffe01ea3bf800
calltrap() at calltrap+0x8/frame 0xfffffe01ea3bf800
--- trap 0x12, rip = 0xffffffff802998e5, rsp = 0xfffffe01ea3bf8d0, rbp = 0xfffffe01ea3bf900 ---
cam_iosched_ticker() at cam_iosched_ticker+0x25/frame 0xfffffe01ea3bf900
softclock_call_cc() at softclock_call_cc+0x13e/frame 0xfffffe01ea3bf9b0
softclock() at softclock+0xb9/frame 0xfffffe01ea3bf9e0
intr_event_execute_handlers() at intr_event_execute_handlers+0xec/frame 0xfffffe01ea3bfa20
ithread_loop() at ithread_loop+0xd6/frame 0xfffffe01ea3bfa70
fork_exit() at fork_exit+0x85/frame 0xfffffe01ea3bfab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe01ea3bfab0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
Uptime: 2h29m20s
Dumping 1165 out of 8055 MB:..2%..11%..21%..31%..42%..51%..61%..71%..81%..91%

PR: 221957
Obtained from: ElectroBSD

[imp]

You listened to my prior feedback. This is perfect now. Thanks!