Specify type alignment for kernel parameters instead of sizeof(long).

The alignment attribute is used to prevent gcc from increasing the
alignment of objects with static extent as an optimisation, something
which would mess up the __setup array stride.

Using __alignof__(struct obs_kernel_param) rather than sizeof(long) is
preferred since it better indicates why it is there and doesn't break
should the type size or alignment change.

Note that on m68k the alignment of struct obs_kernel_param is actually
two and that adding a 1- or 2-byte field to the 12-byte struct would
cause a breakage with the current 4-byte alignment.

Link: https://lore.kernel.org/lkml/20201103175711.10731-1-johan@kernel.orgSigned-off-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NJessica Yu <jeyu@kernel.org>