staging: lustre: remove invariant in cl_io_read_ahead()

This invariant can fail as ci_type is somtimes CIS_INIT.

According to Jinshan Xiong, or of the most recent significant
change to this code, "We should get rid of it."

So do that.

Fixes: 1e1db2a9 ("staging: lustre: clio: Revise read ahead implementation")
Cc: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>