Another uvm_amap panic in -current

[Jared McNeill <jmcneill%invisible.ca@localhost>]

Console output stopped working part-way through the traceback so only 
part of the trapframe is printed.

[ 153620.7406721] panic: kernel diagnostic assertion "anon != NULL && anon->an_ref != 0" failed: file "/home/source/ab/HEAD/src/sys/uvm/uvm_amap.c", line 748
[ 153620.7534934] cpu0: Begin traceback...
[ 153620.7534934] trace fp ffffc008414bfb30
[ 153620.7814018] fp ffffc008414bfb50 vpanic() at ffffc000004adf4c netbsd:vpanic+0x15c
[ 153620.7934242] fp ffffc008414bfbc0 kern_assert() at ffffc000007ccdec netbsd:kern_assert+0x5c
[ 153620.8014956] fp ffffc008414bfc50 amap_wipeout() at ffffc0000040bc94 netbsd:amap_wipeout+0xf4
[ 153620.8315156] fp ffffc008414bfc90 uvm_unmap_detach() at ffffc0000041c454 netbsd:uvm_unmap_detach+0x6c
[ 153620.8453990] fp ffffc008414bfcd0 uvmspace_free() at ffffc0000041ef08 netbsd:uvmspace_free+0xd8
[ 153620.8544258] fp ffffc008414bfd00 exit1() at ffffc00000452bcc netbsd:exit1+0x174
[ 153620.8544258] fp ffffc008414bfdf0 sys_exit() at ffffc00000453498 netbsd:sys_exit+0x38
[ 153620.9044388] fp ffffc008414bfe20 syscall() at ffffc0000008c384 netbsd:syscall+0x18c
[ 153620.9044388] tf ffffc008414bfed0 el0_trap() at ffffc0000008a994 netbsd:el0_trap
[ 153620.9197358] ---- trapframe 0xffffc008414bfed0 (304 bytes) ----
[ 153620.9272872]     pc=0000f4aa7d931478,   spsr=0000000080000000
[ 153620.9272872]    esr=0000000056000001,    far=0000f4aa7cab5048