target: Use FD_MAX_SECTORS/FD_BLOCKSIZE for blockdevs using fileio

We can still see the error reported in

https://patchwork.kernel.org/patch/2338981/

when using fileio backed by a block device.

I'm assuming this will get us past that error (from sbc_parse_cdb),
and also assuming it's OK to have our max_sectors be larger than
the block's queue max hw sectors?

Reported-by: Eric Harney <eharney@redhat.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>