target: Minor cleanups to core_tmr_drain_tmr_list

This patch adds a handful minor cleanups to core_tmr_drain_tmr_list() that
remove an unnecessary NULL check, use list_for_each_entry_safe() instead of
list_entry(), and makes the drain_tmr_list walk use *tmr_p instead of
directly referencing the passed *tmr function parameter.

Signed-off-by: Joern Engel <joern@logfs.org>
Cc: Joern Engel <joern@logfs.org>
Reviewed-by: Roland Dreier <roland@purestorage.com>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>