vmlinux.lds.h: use correct .init.data.* section name

If building with -fdata-sections on riscv, LD_ORPHAN_WARN will warn
similar as below:

riscv64-linux-gnu-ld: warning: orphan section `.init.data.efi_loglevel'
from `./drivers/firmware/efi/libstub/printk.stub.o' being placed in
section `.init.data.efi_loglevel'

I believe this is caused by a a typo:
init.data.* should be .init.data.*

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com> # build
Link: https://lore.kernel.org/r/20230523165502.2592-4-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>