tcmu: allow max block and global max blocks to be settable

Users might have a physical system to a target so they could
have a lot more than 2 gigs of memory they want to devote to
tcmu. OTOH, we could be running in a vm and so a 2 gig
global and 1 gig per dev limit might be too high. This patch
allows the user to specify the limits.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>