lockdebug kernel instacrash when npf enabled

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hi,

I updated an evbarm host from current as of 2018-12-09 to today's
sources. Had some issues with hangs in npf so enabled lockdebug and it
crashed immediately with this message:

Enabling NPF.
[  22.6038371] panic: kernel debugging assertion "pserialize_not_in_read_section()" failed: file "/work/src/sys/kern/kern_mutex.c", line 527
[  22.7529500] cpu0: Begin traceback...
[  22.7976654] 0x99deba54: netbsd:db_panic+0x14
[  22.8465447] 0x99deba6c: netbsd:vpanic+0x194
[  22.8985454] 0x99deba84: netbsd:__aeabi_uldivmod
[  22.9505468] 0x99debb04: netbsd:mutex_enter+0x5f4
[  22.9994280] 0x99debb4c: netbsd:npf_table_lookup+0x134
[  23.0597517] 0x99debb74: netbsd:npf_cop_table+0x70
[  23.1169500] 0x99debba4: netbsd:bpf_filter_ext+0x73c
[  23.1741482] 0x99debc24: netbsd:npf_ruleset_inspect+0x164
[  23.2375871] 0x99debccc: netbsd:npf_packet_handler+0x1cc
[  23.2999868] 0x99debd24: netbsd:pfil_run_hooks+0x128
[  23.3582278] 0x99debe1c: netbsd:ip_output+0x7ac
[  23.4175060] 0x99debe7c: netbsd:ip_forward+0x1a4
[  23.4653506] 0x99debf4c: netbsd:ipintr+0x115c
[  23.5225442] 0x99debfac: netbsd:softint_dispatch+0x114
[  23.5766292] 0x9a729cc4: netbsd:softint_switch+0x58
[  23.6348677] 0x9a729d44: netbsd:irq_entry+0x88
[  23.6920657] 0x9a729db4: netbsd:mutex_enter+0x5bc
[  23.7409489] 0x9a729e24: netbsd:pmap_remove+0x15c
[  23.7960705] 0x9a729e7c: netbsd:uvm_unmap_remove+0x258
[  23.8563886] 0x9a729eac: netbsd:uvmspace_free+0xec
[  23.9135859] 0x9a729f14: netbsd:exit1+0x1a0
[  23.9697447] 0x9a729f34: netbsd:sys_exit+0x3c
[  24.0186247] 0x9a729fac: netbsd:syscall+0x18c