From: liujinbao1 <liujinbao1@xxxxxxxxxx>
Currently, IO can only be ignored when GC_URGENT_HIGH is set,
and the default algorithm used for GC_URGENT_HIGH is greedy.
It gives a way to enable/disable IO aware feature for background
gc, so that we can tune background gc more precisely. e.g.
force to disable IO aware and choose more suitable algorithm
if there are large number of dirty segments.
Signed-off-by: liujinbao1 <liujinbao1@xxxxxxxxxx>