net/mlx4: add iovec-like allocation wrappers

net/mlx4: add iovec-like allocation wrappers

These wrappers implement the ability to allocate room for several disparate
objects as a single contiguous allocation while complying with their
respective alignment constraints.

This is usually more efficient than allocating and freeing them
individually if they are not expected to be reallocated with rte_realloc().

A typical use case is when several objects that cannot be dissociated must
be allocated together, as shown in the following example:

 struct b {
    ...
    struct d *d;
 }

 struct a {
     ...
     struct b *b;
     struct c *c;
 }

 struct mlx4_malloc_vec vec[] = {
     { .size = sizeof(struct a), .addr = &ptr_a, },
     { .size = sizeof(struct b), .addr = &ptr_b, },
     { .size = sizeof(struct c), .addr = &ptr_c, },
     { .size = sizeof(struct d), .addr = &ptr_d, },
 };

 if (!mlx4_mallocv(NULL, vec, RTE_DIM(vec)))
     goto error;

 struct a *a = ptr_a;

 a->b = ptr_b;
 a->c = ptr_c;
 a->b->d = ptr_d;
 ...
 rte_free(a);

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>