sh: vdso: move the sysctl to arch/sh/kernel/vsyscall/vsyscall.c

When CONFIG_SUPERH and CONFIG_VSYSCALL are defined,
vdso_enabled belongs to arch/sh/kernel/vsyscall/vsyscall.c.
So, move it into its own file. To avoid failure when registering
the vdso_table, move the call to register_sysctl_init() into
its own fs_initcall().

Signed-off-by: Kaixiong Yu <yukaixiong@huawei.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>