closures: Change BUG_ON() to WARN_ON()

closures: Change BUG_ON() to WARN_ON()

If a BUG_ON() can be hit in the wild, it shouldn't be a BUG_ON()

For reference, this has popped up once in the CI, and we'll need more
info to debug it:

03240 ------------[ cut here ]------------
03240 kernel BUG at lib/closure.c:21!
03240 kernel BUG at lib/closure.c:21!
03240 Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
03240 Modules linked in:
03240 CPU: 15 PID: 40534 Comm: kworker/u80:1 Not tainted 6.10.0-rc4-ktest-ga56da69799bd #25570
03240 Hardware name: linux,dummy-virt (DT)
03240 Workqueue: btree_update btree_interior_update_work
03240 pstate: 00001005 (nzcv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
03240 pc : closure_put+0x224/0x2a0
03240 lr : closure_put+0x24/0x2a0
03240 sp : ffff0000d12071c0
03240 x29: ffff0000d12071c0 x28: dfff800000000000 x27: ffff0000d1207360
03240 x26: 0000000000000040 x25: 0000000000000040 x24: 0000000000000040
03240 x23: ffff0000c1f20180 x22: 0000000000000000 x21: ffff0000c1f20168
03240 x20: 0000000040000000 x19: ffff0000c1f20140 x18: 0000000000000001
03240 x17: 0000000000003aa0 x16: 0000000000003ad0 x15: 1fffe0001c326974
03240 x14: 0000000000000a1e x13: 0000000000000000 x12: 1fffe000183e402d
03240 x11: ffff6000183e402d x10: dfff800000000000 x9 : ffff6000183e402e
03240 x8 : 0000000000000001 x7 : 00009fffe7c1bfd3 x6 : ffff0000c1f2016b
03240 x5 : ffff0000c1f20168 x4 : ffff6000183e402e x3 : ffff800081391954
03240 x2 : 0000000000000001 x1 : 0000000000000000 x0 : 00000000a8000000
03240 Call trace:
03240  closure_put+0x224/0x2a0
03240  bch2_check_for_deadlock+0x910/0x1028
03240  bch2_six_check_for_deadlock+0x1c/0x30
03240  six_lock_slowpath.isra.0+0x29c/0xed0
03240  six_lock_ip_waiter+0xa8/0xf8
03240  __bch2_btree_node_lock_write+0x14c/0x298
03240  bch2_trans_lock_write+0x6d4/0xb10
03240  __bch2_trans_commit+0x135c/0x5520
03240  btree_interior_update_work+0x1248/0x1c10
03240  process_scheduled_works+0x53c/0xd90
03240  worker_thread+0x370/0x8c8
03240  kthread+0x258/0x2e8
03240  ret_from_fork+0x10/0x20
03240 Code: aa1303e0 d63f0020 a94363f7 17ffff8c (d4210000)
03240 ---[ end trace 0000000000000000 ]---
03240 Kernel panic - not syncing: Oops - BUG: Fatal exception
03240 SMP: stopping secondary CPUs
03241 SMP: failed to stop secondary CPUs 13,15
03241 Kernel Offset: disabled
03241 CPU features: 0x00,00000003,80000008,4240500b
03241 Memory Limit: none
03241 ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]---
03246 ========= FAILED TIMEOUT copygc_torture_no_checksum in 7200s

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>