soc: qcom: smem: Use struct_size()

Use struct_size() instead of hand-writing it, when allocating a structure
with a flex array.

This is less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/f74328551cfab0262ba353f37d047ac74bf616e1.1689194490.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Bjorn Andersson <andersson@kernel.org>