random: fix vgetrandom_opaque_params kernel-doc

struct vgetrandom_opaque_params contains size_of_opaque_state, but its
kernel-doc describes size_per_opaque_state. This leaves the real member
undescribed and adds a nonexistent member to the generated
documentation.

Use the member's actual name.

Fixes: 4ad10a5f5f78 ("random: introduce generic vDSO getrandom() implementation")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>