Buffers must be a multiple of sector_size in length
Buffers must be a multiple of sector_size in length

Before this patch the I/O would fail with an EINVAL, leaving
the developer to wonder what went wrong. This patch logs and
returns a descriptive error message.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

mirage/mirage-block-unix,djs55/mirage-block-unix,samoht/mirage-block-unix