tcm_loop: bump max_sectors

There is not reason to artifically limit max_sectors in tcm_loop, set
it to UINT_MAX to allow stressing the large I/O handling in the target
core using the loopback driver.  Also remove various superflous defines
hiding the values set in the host template.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>