block, bfq: Fix division by zero error on zero wsum

In the Linux kernel, the following vulnerability has been resolved:

block, bfq: Fix division by zero error on zero wsum

When the weighted sum is zero the calculation of limit causes
a division by zero error. Fix this by continuing to the next level.

This was discovered by running as root:

stress-ng --ioprio 0

Fixes divison by error oops:

[ 521.450556] divide error: 0000 [#1] SMP NOPTI
[ 521.450766] CPU: 2 PID: 2684464 Comm: stress-ng-iopri Not tainted 6.2.1-1280.native #1
[ 521.451117] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.1-0-g3208b098f51a-prebuilt.qemu.org 04/01/2014
[ 521.451627] RIP: 0010:bfqq_request_over_limit+0x207/0x400
[ 521.451875] Code: 01 48 8d 0c c8 74 0b 48 8b 82 98 00 00 00 48 8d 0c c8 8b 85 34 ff ff ff 48 89 ca 41 0f af 41 50 48 d1 ea 48 98 48 01 d0 31 d2 <48> f7 f1 41 39 41 48 89 85 34 ff ff ff 0f 8c 7b 01 00 00 49 8b 44
[ 521.452699] RSP: 0018:ffffb1af84eb3948 EFLAGS: 00010046
[ 521.452938] RAX: 000000000000003c RBX: 0000000000000000 RCX: 0000000000000000
[ 521.453262] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffb1af84eb3978
[ 521.453584] RBP: ffffb1af84eb3a30 R08: 0000000000000001 R09: ffff8f88ab8a4ba0
[ 521.453905] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8f88ab8a4b18
[ 521.454224] R13: ffff8f8699093000 R14: 0000000000000001 R15: ffffb1af84eb3970
[ 521.454549] FS: 00005640b6b0b580(0000) GS:ffff8f88b3880000(0000) knlGS:0000000000000000
[ 521.454912] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 521.455170] CR2: 00007ffcbcae4e38 CR3: 00000002e46de001 CR4: 0000000000770ee0
[ 521.455491] PKRU: 55555554
[ 521.455619] Call Trace:
[ 521.455736] <TASK>
[ 521.455837] ? bfq_request_merge+0x3a/0xc0
[ 521.456027] ? elv_merge+0x115/0x140
[ 521.456191] bfq_limit_depth+0xc8/0x240
[ 521.456366] __blk_mq_alloc_requests+0x21a/0x2c0
[ 521.456577] blk_mq_submit_bio+0x23c/0x6c0
[ 521.456766] __submit_bio+0xb8/0x140
[ 521.457236] submit_bio_noacct_nocheck+0x212/0x300
[ 521.457748] submit_bio_noacct+0x1a6/0x580
[ 521.458220] submit_bio+0x43/0x80
[ 521.458660] ext4_io_submit+0x23/0x80
[ 521.459116] ext4_do_writepages+0x40a/0xd00
[ 521.459596] ext4_writepages+0x65/0x100
[ 521.460050] do_writepages+0xb7/0x1c0
[ 521.460492] __filemap_fdatawrite_range+0xa6/0x100
[ 521.460979] file_write_and_wait_range+0xbf/0x140
[ 521.461452] ext4_sync_file+0x105/0x340
[ 521.461882] __x64_sys_fsync+0x67/0x100
[ 521.462305] ? syscall_exit_to_user_mode+0x2c/0x1c0
[ 521.462768] do_syscall_64+0x3b/0xc0
[ 521.463165] entry_SYSCALL_64_after_hwframe+0x5a/0xc4
[ 521.463621] RIP: 0033:0x5640b6c56590
[ 521.464006] Code: 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 80 3d 71 70 0e 00 00 74 17 b8 4a 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 48 c3 0f 1f 80 00 00 00 00 48 83 ec 18 89 7c