kernel: Modify the signature of k_mem_slab_free()

Modify the signature of the k_mem_slab_free() function with a new one,
replacing the old void **mem with void *mem as a parameter.

The following function:
void k_mem_slab_free(struct k_mem_slab *slab, void **mem);

has the wrong signature. mem is only used as a regular pointer, so there
is no need to use a double-pointer. The correct signature should be:
void k_mem_slab_free(struct k_mem_slab *slab, void *mem);

The issue with the current signature, although functional, is that it is
extremely confusing. I myself, a veteran Zephyr developer, was confused
by this parameter when looking at it recently.

All in-tree uses of the function have been adapted.

Fixes #61888.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>