ipfilter: Avoid stopping with a lock held

Avoid calling _callout_stop_safe with a non-sleepable lock held when
detaching by initializing callout_init_rw() with CALLOUT_SHAREDLOCK.

It avoids the following WITNESS warning when stopping the service:

    # service ipfilter stop
    calling _callout_stop_safe with the following non-sleepable locks held:
    shared rw ipf filter load/unload mutex (ipf filter load/unload mutex) r = 0 (0xffff0000417c7530) locked @ /usr/src/sys/netpfil/ipfilter/netinet/fil.c:7926
    stack backtrace:
    #0 0xffff00000052d394 at witness_debugger+0x60
    #1 0xffff00000052e620 at witness_warn+0x404
    #2 0xffff0000004d4ffc at _callout_stop_safe+0x8c
    #3 0xffff0000f7236674 at ipfdetach+0x3c
    #4 0xffff0000f723fa4c at ipf_ipf_ioctl+0x788
    #5 0xffff0000f72367e0 at ipfioctl+0x144
    #6 0xffff00000034abd8 at devfs_ioctl+0x100
    #7 0xffff0000005c66a0 at vn_ioctl+0xbc
    #8 0xffff00000034b2cc at devfs_ioctl_f+0x24
    #9 0xffff0000005331ec at kern_ioctl+0x2e0
    #10 0xffff000000532eb4 at sys_ioctl+0x140
    #11 0xffff000000880480 at do_el0_sync+0x604
    #12 0xffff0000008579ac at handle_el0_sync+0x4c

PR:		282478
Suggested by:	markj
Reviewed by:	cy
Approved by:	emaste (mentor)
MFC after:	1 week

sys/netpfil/ipfilter/netinet/ip_fil_freebsd.c

@@ -181,7 +181,7 @@ ipf_timer_func(void *arg)
 #if 0
 		softc->ipf_slow_ch = timeout(ipf_timer_func, softc, hz/2);
 #endif
-		callout_init(&softc->ipf_slow_ch, 1);
+		callout_init_rw(&softc->ipf_slow_ch, &softc->ipf_global.ipf_lk, CALLOUT_SHAREDLOCK);
 		callout_reset(&softc->ipf_slow_ch,
 			(hz / IPF_HZ_DIVIDE) * IPF_HZ_MULT,
 			ipf_timer_func, softc);
@@ -221,7 +221,7 @@ ipfattach(ipf_main_softc_t *softc)
 	softc->ipf_slow_ch = timeout(ipf_timer_func, softc,
 				     (hz / IPF_HZ_DIVIDE) * IPF_HZ_MULT);
 #endif
-	callout_init(&softc->ipf_slow_ch, 1);
+	callout_init_rw(&softc->ipf_slow_ch, &softc->ipf_global.ipf_lk, CALLOUT_SHAREDLOCK);
 	callout_reset(&softc->ipf_slow_ch, (hz / IPF_HZ_DIVIDE) * IPF_HZ_MULT,
 		ipf_timer_func, softc);
 	return (0);