crimson/osd/osd_operations/snaptrim_event: fix lifetime on finally()

crimson/osd/osd_operations/snaptrim_event: fix lifetime on finally()

Sanitized backtrace:
```
DEBUG 2023-11-14 15:23:50,871 [shard 0] osd - snaptrim_event(id=10610, detail=SnapTrimEvent(pgid=16.1a snapid=a needs_pause=0)): interrupted crimson::common::actingset_changed (acting set changed)

    #0 0x5653c613c071 in seastar::shared_mutex::unlock() (/usr/bin/ceph-osd+0x1ed27071)
    #1 0x5653c8670acf in auto seastar::futurize_invoke<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::ExitBarrier<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::BlockingEvent::Trigger<crimson::osd::SnapTrimEvent> >::exit()::{lambda()#1}&>(crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::ExitBarrier<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::BlockingEvent::Trigger<crimson::osd::SnapTrimEvent> >::exit()::{lambda()#1}&) (/usr/bin/ceph-osd+0x2125bacf)
    #2 0x5653c8670e22 in _ZN7seastar20noncopyable_functionIFNS_6futureIvEEvEE17direct_vtable_forIZNS2_4thenIZN7crimson23OrderedConcurrentPhaseTINS7_3osd13SnapTrimEvent9WaitSubopEE11ExitBarrierINSC_13BlockingEvent7TriggerISA_EEE4exitEvEUlvE_S2_EET0_OT_EUlDpOT_E_E4callEPKS4_ (/usr/bin/ceph-osd+0x2125be22)

freed by thread T1 here:
    #0 0x7f10628b73cf in operator delete(void*, unsigned long) (/lib64/libasan.so.6+0xb73cf)
    #1 0x5653c8794bff in crimson::osd::SnapTrimEvent::~SnapTrimEvent() (/usr/bin/ceph-osd+0x2137fbff)

previously allocated by thread T1 here:
    #0 0x7f10628b6367 in operator new(unsigned long) (/lib64/libasan.so.6+0xb6367)

SUMMARY: AddressSanitizer: heap-use-after-free (/usr/bin/ceph-osd+0x1ed27071) in seastar::shared_mutex::unlock()
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>