Overnight lockdebug did find something:
login: [ 1908.3939406] Mutex error: mutex_vector_enter,504: spinout
[ 1908.3939406] lock address : 0x0000000090b79074 type : spin
[ 1908.3939406] initialized : 0x000000008041601c
[ 1908.3939406] shared holds : 0 exclusive: 1
[ 1908.3939406] shares wanted: 0 exclusive: 1
[ 1908.3939406] current cpu : 0 last held: 1
[ 1908.3939406] current lwp : 0x0000000091fc3760 last held: 0x0000000091fc26e0
[ 1908.3939406] last locked* : 0x0000000080416668 unlocked : 0x00000000804169e8
[ 1908.3939406] owner field : 0x0000000000010500 wait/spin: 0/1
[ 1908.4626458] panic: LOCKDEBUG: Mutex error: mutex_vector_enter,504: spinout
[ 1908.4626458] cpu0: Begin traceback...
[ 1908.4626458] 0x9e4a192c: netbsd:db_panic+0x14
[ 1908.4626458] 0x9e4a1944: netbsd:vpanic+0x194
[ 1908.4626458] 0x9e4a195c: netbsd:snprintf
[ 1908.4626458] 0x9e4a199c: netbsd:lockdebug_more
[ 1908.4626458] 0x9e4a19d4: netbsd:lockdebug_abort+0xc0
[ 1908.4626458] 0x9e4a19f4: netbsd:mutex_abort+0x34
[ 1908.4626458] 0x9e4a1a64: netbsd:mutex_enter+0x580
[ 1908.4626458] 0x9e4a1abc: netbsd:pool_get+0x70
[ 1908.4626458] 0x9e4a1b0c: netbsd:pool_cache_get_slow+0x1f4
[ 1908.4626458] 0x9e4a1b5c: netbsd:pool_cache_get_paddr+0x288
[ 1908.4626458] 0x9e4a1b7c: netbsd:m_clget+0x34
[ 1908.4626458] 0x9e4a1bdc: netbsd:dwc_gmac_intr+0x194
[ 1908.4626458] 0x9e4a1bf4: netbsd:gic_fdt_intr+0x2c
[ 1908.4626458] 0x9e4a1c1c: netbsd:pic_dispatch+0x110
[ 1908.4626458] 0x9e4a1c7c: netbsd:armgic_irq_handler+0xf4
[ 1908.4626458] 0x9e4a1db4: netbsd:irq_entry+0x68
[ 1908.4626458] 0x9e4a1dec: netbsd:tcp_send_wrapper+0x9c
[ 1908.4626458] 0x9e4a1e84: netbsd:sosend+0x6fc
[ 1908.4626458] 0x9e4a1eac: netbsd:soo_write+0x3c
[ 1908.4626458] 0x9e4a1f04: netbsd:dofilewrite+0x7c
[ 1908.4626458] 0x9e4a1f34: netbsd:sys_write+0x5c
[ 1908.4626458] 0x9e4a1fac: netbsd:syscall+0x12c
[ 1908.4626458] cpu0: End traceback...