srcu: Delegate work to the boot cpu if using SRCU_SIZE_SMALL

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

srcu: Delegate work to the boot cpu if using SRCU_SIZE_SMALL

Commit 994f706872e6 ("srcu: Make Tree SRCU able to operate without
snp_node array") assumes that cpu 0 is always online. However, there
really are situations when some other CPU is the boot CPU, for example,
when booting a kdump kernel with the maxcpus=1 boot parameter.

On PowerPC, the kdump kernel can hang as follows:
...
[ 1.740036] systemd[1]: Hostname set to <xyz.com>
[ 243.686240] INFO: task systemd:1 blocked for more than 122 seconds.
[ 243.686264] Not tainted 6.1.0-rc1 #1
[ 243.686272] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 243.686281] task:systemd state:D stack:0 pid:1 ppid:0 flags:0x00042000
[ 243.686296] Call Trace:
[ 243.686301] [c000000016657640] [c000000016657670] 0xc000000016657670 (unreliable)
[ 243.686317] [c000000016657830] [c00000001001dec0] __switch_to+0x130/0x220
[ 243.686333] [c000000016657890] [c000000010f607b8] __schedule+0x1f8/0x580
[ 243.686347] [c000000016657940] [c000000010f60bb4] schedule+0x74/0x140
[ 243.686361] [c0000000166579b0] [c000000010f699b8] schedule_timeout+0x168/0x1c0
[ 243.686374] [c000000016657a80] [c000000010f61de8] __wait_for_common+0x148/0x360
[ 243.686387] [c000000016657b20] [c000000010176bb0] __flush_work.isra.0+0x1c0/0x3d0
[ 243.686401] [c000000016657bb0] [c0000000105f2768] fsnotify_wait_marks_destroyed+0x28/0x40
[ 243.686415] [c000000016657bd0] [c0000000105f21b8] fsnotify_destroy_group+0x68/0x160
[ 243.686428] [c000000016657c40] [c0000000105f6500] inotify_release+0x30/0xa0
[ 243.686440] [c000000016657cb0] [c0000000105751a8] __fput+0xc8/0x350
[ 243.686452] [c000000016657d00] [c00000001017d524] task_work_run+0xe4/0x170
[ 243.686464] [c000000016657d50] [c000000010020e94] do_notify_resume+0x134/0x140
[ 243.686478] [c000000016657d80] [c00000001002eb18] interrupt_exit_user_prepare_main+0x198/0x270
[ 243.686493] [c000000016657de0] [c00000001002ec60] syscall_exit_prepare+0x70/0x180
[ 243.686505] [c000000016657e10] [c00000001000bf7c] system_call_vectored_common+0xfc/0x280
[ 243.686520] --- interrupt: 3000 at 0x7fffa47d5ba4
[ 243.686528] NIP: 00007fffa47d5ba4 LR: 0000000000000000 CTR: 0000000000000000
[ 243.686538] REGS: c000000016657e80 TRAP: 3000 Not tainted (6.1.0-rc1)
[ 243.686548] MSR: 800000000000d033 <SF,EE,PR,ME,IR,DR,RI,LE> CR: 42044440 XER: 00000000
[ 243.686572] IRQMASK: 0
[ 243.686572] GPR00: 0000000000000006 00007ffffa606710 00007fffa48e7200 0000000000000000
[ 243.686572] GPR04: 0000000000000002 000000000000000a 0000000000000000 0000000000000001
[ 243.686572] GPR08: 000001000c172dd0 0000000000000000 0000000000000000 0000000000000000
[ 243.686572] GPR12: 0000000000000000 00007fffa4ff4bc0 0000000000000000 0000000000000000
[ 243.686572] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 243.686572] GPR20: 0000000132dfdc50 000000000000000e 0000000000189375 0000000000000000
[ 243.686572] GPR24: 00007ffffa606ae0 0000000000000005 000001000c185490 000001000c172570
[ 243.686572] GPR28: 000001000c172990 000001000c184850 000001000c172e00 00007fffa4fedd98
[ 243.686683] NIP [00007fffa47d5ba4] 0x7fffa47d5ba4
[ 243.686691] LR [0000000000000000] 0x0
[ 243.686698] --- interrupt: 3000
[ 243.686708] INFO: task kworker/u16:1:24 blocked for more than 122 seconds.
[ 243.686717] Not tainted 6.1.0-rc1 #1
[ 243.686724] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 243.686733] task:kworker/u16:1 state:D stack:0 pid:24 ppid:2 flags:0x00000800
[ 243.686747] Workqueue: events_unbound fsnotify_mark_destroy_workfn
[ 243.686758] Call Trace:
[ 243.686762] [c0000000166736e0] [c00000004fd91000] 0xc00000004fd91000 (unreliable)
[ 243.686775] [c0000000166738d0] [c00000001001dec0] __switch_to+0x130/0x220
[ 243.686788] [c000000016673930] [c000000010f607b8] __schedule+0x1f8/0x
---truncated---