riscv: drop __init from vec_check_unaligned_access_speed_all_cpus

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

riscv: drop __init from vec_check_unaligned_access_speed_all_cpus

This function runs within a kthread and need not necessarily finish
before system finishes boot and free_initmem() unmaps the .init.text
section. This function makes calls to SBI for probing unaligned access
speed, and if this is slow for some reason (say some debug prints were
added to SBI), the kthread can still be running at this point and result
in an instruction page fault when trying to fetch from the freed region.

[ 25.642087] Unable to handle kernel paging request at virtual address ffffffff80a04ef8
[ 25.646694] Current vec_check_unali pgtable: 4K pagesize, 48-bit VAs, pgdp=0x00004000316e9000
[ 25.653170] [ffffffff80a04ef8] pgd=000010004be7e401, p4d=000010004be7e401, pud=000010004be7e001, pmd=000010000c3000e3
[ 25.661244] Oops [#1]
[ 25.662997] Modules linked in:
[ 25.665357] CPU: 3 UID: 0 PID: 42 Comm: vec_check_unali Not tainted 7.0.0-tt-blackhole-asrinivasan-00007-g30ff73f18211 #570 PREEMPTLAZY
[ 25.674669] Hardware name: Tenstorrent Blackhole (DT)
[ 25.678545] epc : vec_check_unaligned_access_speed_all_cpus+0x18/0x2c
[ 25.683458] ra : vec_check_unaligned_access_speed_all_cpus+0x18/0x2c
[ 25.688372] epc : ffffffff80a04ef8 ra : ffffffff80a04ef8 sp : ffff8f8000203e20
[ 25.693874] gp : ffffffff814dc168 tp : ffffaf8001ad9900 t0 : 0000000000000000
[ 25.699401] t1 : fffffffffffffff0 t2 : ffffaf8001ad9a10 s0 : ffff8f8000203e30
[ 25.704912] s1 : ffffaf80018dc780 a0 : 0000000000000000 a1 : 0000000000000002
[ 25.710407] a2 : 00000000000001f0 a3 : 0000000000000018 a4 : 0000000000000000
[ 25.715917] a5 : 0000000000000000 a6 : ffffaf8001c03d98 a7 : ffffaf8001c03e30
[ 25.721419] s2 : ffff8f8000023c98 s3 : ffffaf8001aa1240 s4 : ffffffff80a04ee0
[ 25.726937] s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
[ 25.732450] s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000
[ 25.737944] s11: 0000000000000000 t3 : 0000000000000002 t4 : 0000000000000402
[ 25.743481] t5 : 0000000000000040 t6 : 0000000000000004 ssp : 0000000000000000
[ 25.749024] status: 0000000200000120 badaddr: ffffffff80a04ef8 cause: 000000000000000c
[ 25.755060] [<ffffffff80a04ef8>] vec_check_unaligned_access_speed_all_cpus+0x18/0x2c
[ 25.760964] [<ffffffff80047a10>] kthread+0xd8/0xfc
[ 25.764660] [<ffffffff80010c48>] ret_from_fork_kernel+0x18/0x1c4
[ 25.769220] [<ffffffff80895fe6>] ret_from_fork_kernel_asm+0x16/0x18
[ 25.774018] Code: cccc cccc cccc cccc cccc cccc cccc cccc cccc cccc (cccc) cccc

Drop __init from its signature so that this doesn't happen.