fs/fat: Fix 'CACHE: Misaligned operation at range' warnings

The 'block' field of fat_itr needs to be properly aligned for DMA and
while it does have '__aligned(ARCH_DMA_MINALIGN)', the fat_itr structure
itself needs to be properly aligned as well.

While at it use malloc_cache_aligned() for the other aligned allocations
in the file as well.

Fixes: 2460098c

 ("fs/fat: Reduce stack usage")
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Tom Rini <trini@konsulko.com>