riscv: vdso: fix NULL deference in vdso_join_timens() when vfork

Testing tools/testing/selftests/timens/vfork_exec.c got below
kernel log:

[    6.838454] Unable to handle kernel access to user memory without uaccess routines at virtual address 0000000000000020
[    6.842255] Oops [#1]
[    6.842871] Modules linked in:
[    6.844249] CPU: 1 PID: 64 Comm: vfork_exec Not tainted 6.0.0-rc3-rt15+ #8
[    6.845861] Hardware name: riscv-virtio,qemu (DT)
[    6.848009] epc : vdso_join_timens+0xd2/0x110
[    6.850097]  ra : vdso_join_timens+0xd2/0x110
[    6.851164] epc : ffffffff8000635c ra : ffffffff8000635c sp : ff6000000181fbf0
[    6.852562]  gp : ffffffff80cff648 tp : ff60000000fdb700 t0 : 3030303030303030
[    6.853852]  t1 : 0000000000000030 t2 : 3030303030303030 s0 : ff6000000181fc40
[    6.854984]  s1 : ff60000001e6c000 a0 : 0000000000000010 a1 : ffffffff8005654c
[    6.856221]  a2 : 00000000ffffefff a3 : 0000000000000000 a4 : 0000000000000000
[    6.858114]  a5 : 0000000000000000 a6 : 0000000000000008 a7 : 0000000000000038
[    6.859484]  s2 : ff60000001e6c068 s3 : ff6000000108abb0 s4 : 0000000000000000
[    6.860751]  s5 : 0000000000001000 s6 : ffffffff8089dc40 s7 : ffffffff8089dc38
[    6.862029]  s8 : ffffffff8089dc30 s9 : ff60000000fdbe38 s10: 000000000000005e
[    6.863304]  s11: ffffffff80cc3510 t3 : ffffffff80d1112f t4 : ffffffff80d1112f
[    6.864565]  t5 : ffffffff80d11130 t6 : ff6000000181fa00
[    6.865561] status: 0000000000000120 badaddr: 0000000000000020 cause: 000000000000000d
[    6.868046] [<ffffffff8008dc94>] timens_commit+0x38/0x11a
[    6.869089] [<ffffffff8008dde8>] timens_on_fork+0x72/0xb4
[    6.870055] [<ffffffff80190096>] begin_new_exec+0x3c6/0x9f0
[    6.871231] [<ffffffff801d826c>] load_elf_binary+0x628/0x1214
[    6.872304] [<ffffffff8018ee7a>] bprm_execve+0x1f2/0x4e4
[    6.873243] [<ffffffff8018f90c>] do_execveat_common+0x16e/0x1ee
[    6.874258] [<ffffffff8018f9c8>] sys_execve+0x3c/0x48
[    6.875162] [<ffffffff80003556>] ret_from_syscall+0x0/0x2
[    6.877484] ---[ end trace 0000000000000000 ]---

This is because the mm->context.vdso_info is NULL in vfork case. From
another side, mm->context.vdso_info either points to vdso info
for RV64 or vdso info for compat, there's no need to bloat riscv's
mm_context_t, we can handle the difference when setup the additional
page for vdso.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
Fixes: 3092eb45

 ("riscv: compat: vdso: Add setup additional pages implementation")
Link: https://lore.kernel.org/r/20220924070737.3048-1-jszhang@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>