riscv: Flush current cpu icache before other cpus

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

riscv: Flush current cpu icache before other cpus

On SiFive Unmatched, I recently fell onto the following BUG when booting:

[ 0.000000] ftrace: allocating 36610 entries in 144 pages
[ 0.000000] Oops - illegal instruction [#1]
[ 0.000000] Modules linked in:
[ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 5.13.1+ #5
[ 0.000000] Hardware name: SiFive HiFive Unmatched A00 (DT)
[ 0.000000] epc : riscv_cpuid_to_hartid_mask+0x6/0xae
[ 0.000000] ra : __sbi_rfence_v02+0xc8/0x10a
[ 0.000000] epc : ffffffff80007240 ra : ffffffff80009964 sp : ffffffff81803e10
[ 0.000000] gp : ffffffff81a1ea70 tp : ffffffff8180f500 t0 : ffffffe07fe30000
[ 0.000000] t1 : 0000000000000004 t2 : 0000000000000000 s0 : ffffffff81803e60
[ 0.000000] s1 : 0000000000000000 a0 : ffffffff81a22238 a1 : ffffffff81803e10
[ 0.000000] a2 : 0000000000000000 a3 : 0000000000000000 a4 : 0000000000000000
[ 0.000000] a5 : 0000000000000000 a6 : ffffffff8000989c a7 : 0000000052464e43
[ 0.000000] s2 : ffffffff81a220c8 s3 : 0000000000000000 s4 : 0000000000000000
[ 0.000000] s5 : 0000000000000000 s6 : 0000000200000100 s7 : 0000000000000001
[ 0.000000] s8 : ffffffe07fe04040 s9 : ffffffff81a22c80 s10: 0000000000001000
[ 0.000000] s11: 0000000000000004 t3 : 0000000000000001 t4 : 0000000000000008
[ 0.000000] t5 : ffffffcf04000808 t6 : ffffffe3ffddf188
[ 0.000000] status: 0000000200000100 badaddr: 0000000000000000 cause: 0000000000000002
[ 0.000000] [<ffffffff80007240>] riscv_cpuid_to_hartid_mask+0x6/0xae
[ 0.000000] [<ffffffff80009474>] sbi_remote_fence_i+0x1e/0x26
[ 0.000000] [<ffffffff8000b8f4>] flush_icache_all+0x12/0x1a
[ 0.000000] [<ffffffff8000666c>] patch_text_nosync+0x26/0x32
[ 0.000000] [<ffffffff8000884e>] ftrace_init_nop+0x52/0x8c
[ 0.000000] [<ffffffff800f051e>] ftrace_process_locs.isra.0+0x29c/0x360
[ 0.000000] [<ffffffff80a0e3c6>] ftrace_init+0x80/0x130
[ 0.000000] [<ffffffff80a00f8c>] start_kernel+0x5c4/0x8f6
[ 0.000000] ---[ end trace f67eb9af4d8d492b ]---
[ 0.000000] Kernel panic - not syncing: Attempted to kill the idle task!
[ 0.000000] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

While ftrace is looping over a list of addresses to patch, it always failed
when patching the same function: riscv_cpuid_to_hartid_mask. Looking at the
backtrace, the illegal instruction is encountered in this same function.
However, patch_text_nosync, after patching the instructions, calls
flush_icache_range. But looking at what happens in this function:

flush_icache_range -> flush_icache_all
-> sbi_remote_fence_i
-> __sbi_rfence_v02
-> riscv_cpuid_to_hartid_mask

The icache and dcache of the current cpu are never synchronized between the
patching of riscv_cpuid_to_hartid_mask and calling this same function.

So fix this by flushing the current cpu's icache before asking for the other
cpus to do the same.