wapbl + discard panic

[Jeff Rizzo <riz%tastylime.net@localhost>]

I'm guessing at the moment the answer is "don't do that," but it looks 
like wapbl and discard aren't playing nice together:

panic: kernel diagnostic assertion "rw_lock_held(&wl->wl_rwlock)" 
failed: file "/home/riz/src/sys/kern/vfs_wapbl.c", line 1715
Stopped in pid 0.54 (system) at netbsd:cpu_Debugger+0x4: bx      r14
db{0}> bt
0xbfaffe34: netbsd:vpanic+0xc
0xbfaffe4c: netbsd:__udivmoddi4
0xbfaffe74: netbsd:wapbl_jlock_assert+0x54
0xbfaffe9c: netbsd:wapbl_add_buf+0x3c
0xbfaffebc: netbsd:bdwrite+0xc4
0xbfafff1c: netbsd:ffs_blkfree_cg+0x190
0xbfafff4c: netbsd:ffs_blkfree_td+0x60
0xbfafff7c: netbsd:ffs_discardcb+0x64
0xbfafffac: netbsd:workqueue_worker+0x84
db{0}>