mempool: make sure max block size isn't smaller than minimum allowed

If maxsize is smaller than _MPOOL_MINBLK, then Z_MPOOL_LVLS() will be 0.
That means the loop in z_sys_mem_pool_base_init() that initializes the
block free list for the nonexistent level 0 will corrupt whatever memory
at the location the zero-sized struct sys_mem_pool_lvl array was
located. And the corruption happens to be done with a perfectly legit
memory pool block address which makes for really nasty bugs to solve.

This is more likely on 64-bit systems due to _MPOOL_MINBLK being twice
the size of 32-bit systems.

Let's prevent that with a build-time assertion on maxsize when defining
a memory pool, and adjust the affected test accordingly.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

include/sys/mempool.h

@@ -47,6 +47,7 @@ struct sys_mem_pool_block {
  * @param section Destination binary section for pool data
  */
 #define SYS_MEM_POOL_DEFINE(name, ignored, minsz, maxsz, nmax, align, section) \
+	BUILD_ASSERT(WB_UP(maxsz) >= _MPOOL_MINBLK);			\
 	char __aligned(WB_UP(align)) Z_GENERIC_SECTION(section)		\
 		_mpool_buf_##name[WB_UP(maxsz) * nmax			\
 				  + _MPOOL_BITS_SIZE(maxsz, minsz, nmax)]; \