logging: init strdup mem_slab with aligned buffer size

We are using a non-32-bit aligned value as the block size
when initializing the strdup mem_slab for Logger.

When the non-aligned pointer is being freed, it's converted
to a u32_t and this generates the following:
***** USAGE FAULT *****
  Unaligned memory access

Let's use the aligned buffer size as the block size instead.

Signed-off-by: Michael Scott <mike@foundries.io>