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:

1. 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