spl: fit: Allocate buffers aligned to cache line size

Allocate memory for buffers at a cache-line boundary to avoid
misaligned buffer address for subsequent reads. This avoids an
additional sector-based memory copy in the fat file system driver:

FAT: Misaligned buffer address (...)

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>