can: netlink: refactor can_validate_bittiming()

can: netlink: refactor can_validate_bittiming()

Whenever can_validate_bittiming() is called, it is always preceded by
some boilerplate code which was copy pasted all over the place. Move
that repeated code directly inside can_validate_bittiming().

Finally, the mempcy() is not needed: the nla attributes are four bytes
aligned which is just enough for struct can_bittiming. Add a
static_assert() to document that the alignment is correct and just use
the pointer returned by nla_data() as-is.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250923-canxl-netlink-prep-v4-4-e720d28f66fe@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>