reviving ZFS in broken sm_start+sm_size state

Dear colleagues,

after some (AFAIR clean) reboot of current with ZFS-on-root I had (OCRed from 
mobile photo but hopefully good enough) unbootable system with the following 
panic:

--- 8< ---
panic: VERIFY3U(entry_offset, <, sm->sm_start + sm->sm_size) failed (1847270282567680 < 92341796864) 

cpuid = 2
time = 1756738203
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0149c856d0
vpanic at vpanic+0x136/frame 0xfffffe8149c85800
spl_panic at spl_panic+0x3a/frame 0xfffffe0149c85860
space_map_iterate() at space_map_iterate+0x3b1/frame 0xfffffe0149c85920
space_map_load_length() at space_map_load_length+0x5f/frame 0xfffffe8149c85970 
metaslab_load() at metaslab_load+0x529/frame 0xfffffe8149c85a40
metaslab_activate() at metaslab_activate+0x46/frame 0xfffffe8149c85a88
metaslab_alloc_dva_range() at metaslab_alloc_dva_range+0x7f9/frame 0xfffffe0149c85bb0 
metaslab_alloc_range() at metaslab_alloc_range+8x2c2/frame 8xfffffe8149c85c70 
metaslab_alloc() at metaslab_allo 
zio_dva_allocate() at 0xfffffe0149c85cc0
zio_execute() at zio iraframe 0xfffffe0149c85e10/frame 0xfffffe0149c85e40
taskqueue_run_locked) at taskqueue_run_locked+0x1c2/frame 0xfffffe0149c85ec0 
taskqueue_thread_loop() at taskqueue_thread_loop+0xd3/frame 0xfffffe0149c85ef0 
fork_exit() at fork_exit+0x82/frame 0xfffffe0149c85f30
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0149c85f30 
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 0 tid 101011]
Stopped at
 --- 8< ---

attempts to boot from last snapshot and/or trying to boot from PRERELEASE and 
zpool import lead to exactly the same results, even with different '-F' 
options:

pool *seems* to be importable but actually isn't due to mad entry_offset as I 
can see from source

any hints how could I resolve this?  the pool content itself is not **very** 
important, but avoiding recreation would be nice

thanks in advance!


-- 
Sincerely,
D.Marck                                                          [MCK-RIPE]
[ FreeBSD committer:                                    marck@FreeBSD.org ]
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*** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- woozle@woozle.net ***
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