commit a1e1774c upstream.

When compiled with CONFIG_DEBUG_SG set, uninitialized SGL leads
to BUG() in compare_and_write_callback().
Signed-off-by: Martin Svec <martin.svec@zoner.cz>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>