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

This is less verbose, more robust and more informative.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/f4618a67d5ae0a30eb3f2b4558c8cc790feed79a.1690044376.git.christophe.jaillet@wanadoo.frSigned-off-by: Leon Romanovsky <leon@kernel.org>