bcachefs: Fix bkey_nocow_lock()

bcachefs: Fix bkey_nocow_lock()

This fixes an assertion pop in nocow_locking.c

00243 kernel BUG at fs/bcachefs/nocow_locking.c:41!
00243 Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
00243 Modules linked in:
00243 Hardware name: linux,dummy-virt (DT)
00243 pstate: 60001005 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
00244 pc : bch2_bucket_nocow_unlock (/home/testdashboard/linux-7/fs/bcachefs/nocow_locking.c:41)
00244 lr : bkey_nocow_lock (/home/testdashboard/linux-7/fs/bcachefs/data_update.c:79)
00244 sp : ffffff80c82373b0
00244 x29: ffffff80c82373b0 x28: ffffff80e08958c0 x27: ffffff80e0880000
00244 x26: ffffff80c8237a98 x25: 00000000000000a0 x24: ffffff80c8237ab0
00244 x23: 00000000000000c0 x22: 0000000000000008 x21: 0000000000000000
00244 x20: ffffff80c8237a98 x19: 0000000000000018 x18: 0000000000000000
00244 x17: 0000000000000000 x16: 000000000000003f x15: 0000000000000000
00244 x14: 0000000000000008 x13: 0000000000000018 x12: 0000000000000000
00244 x11: 0000000000000000 x10: ffffff80e0880000 x9 : ffffffc0803ac1a4
00244 x8 : 0000000000000018 x7 : ffffff80c8237a88 x6 : ffffff80c8237ab0
00244 x5 : ffffff80e08988d0 x4 : 00000000ffffffff x3 : 0000000000000000
00244 x2 : 0000000000000004 x1 : 0003000000000d1e x0 : ffffff80e08988c0
00244 Call trace:
00244 bch2_bucket_nocow_unlock (/home/testdashboard/linux-7/fs/bcachefs/nocow_locking.c:41)
00245 bch2_data_update_init (/home/testdashboard/linux-7/fs/bcachefs/data_update.c:627 (discriminator 1))
00245 promote_alloc.isra.0 (/home/testdashboard/linux-7/fs/bcachefs/io_read.c:242 /home/testdashboard/linux-7/fs/bcachefs/io_read.c:304)
00245 __bch2_read_extent (/home/testdashboard/linux-7/fs/bcachefs/io_read.c:949)
00246 __bch2_read (/home/testdashboard/linux-7/fs/bcachefs/io_read.c:1215)
00246 bch2_direct_IO_read (/home/testdashboard/linux-7/fs/bcachefs/fs-io-direct.c:132)
00246 bch2_read_iter (/home/testdashboard/linux-7/fs/bcachefs/fs-io-direct.c:201)
00247 aio_read.constprop.0 (/home/testdashboard/linux-7/fs/aio.c:1602)
00247 io_submit_one.constprop.0 (/home/testdashboard/linux-7/fs/aio.c:2003 /home/testdashboard/linux-7/fs/aio.c:2052)
00248 __arm64_sys_io_submit (/home/testdashboard/linux-7/fs/aio.c:2111 /home/testdashboard/linux-7/fs/aio.c:2081 /home/testdashboard/linux-7/fs/aio.c:2081)
00248 invoke_syscall.constprop.0 (/home/testdashboard/linux-7/arch/arm64/include/asm/syscall.h:61 /home/testdashboard/linux-7/arch/arm64/kernel/syscall.c:54)
00248 ========= FAILED TIMEOUT tiering_variable_buckets_replicas in 1200s

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