locking against myself in unionfs code

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I got a "loking against myself" panic when writing to a mount_union'ed fs:

Reader / writer lock error: rw_vector_enter,357: locking against myself
lock address : ffff8498eBe4480
current cpu  :			2
current lwp : 0xffff849875f30a00
owner/count : 0xffff849875f30a00 flags	: 0x000000000000004
panic: lock error: Reader / writer lock: ru vector_enter, 357: locking against myself: lock 0xffff8498e8e44f80 cpu 2 lwp 0xffff849875f30a00
cpu2: Begin traceback...
vpanic() at netbsd:vpanic+0x183
panic() at netbsd:panic+0x3c
lockdebug_abort() at netbsd:lockdebug_abort+0x114
rw_vector_enter() at netbsd:rw_vector_enter+0x1f6
genfs_lock() at netbsd:genfs_lock+0x80
VOP_LOCK() at netbsd:VOP_LOCK+0xb3
vn_lock) at netbsd:vn_lock+0x22
union_lookup1() at netbsd:union_lookup1+0x68
union_lookup() at netbsd:union_lookup+0xa4
VOP_LOOKUP() at netbsd:VOP_LOOKUP+0x3d
relookup() at netbsd:relookup+0x42
union_link() at netbsd:union_link+0x125
VOP_LINK() at netbsd:VOP_LINK+0x46
do_sys_linkat() at netbsd:do_sys_linkat+0x20a
sys_link() at netbsd:sys_link+0x28
syscall() at netbsd:syscall+0x1fc
--- syscall (number 9) ---
netbsd:syscall+0x1fc:
cpu2: End traceback...
fatal breakpoint trap in supervisor mode
trap type 1 code 0 rip 0xffffffff80232795 cs 0x8 rf lags 0x202 cr2 Oxffffd904bfba000 ilevel 0 rsp Oxffffd9051c67b910
curlwp 0xffff849875f30a00 pid 22788.22788 lowest kstack 0xffff49051c6772c0
Stopped in pid 22788.22788 (python3.10)