By David Shepardson, Valerie Insinna and Tim Hepher (Reuters) -U.S. regulators on Saturday temporarily grounded 171 Boeing 737 MAX 9 jetliners for safety checks following a cabin panel blowout that forced a new Alaska Airlines jet carrying passengers to make an emergency landing. A piece of fuselage tore off the left side of the jet as it climbed following takeoff from Portland, Oregon, en route to Ontario, California, on Friday, forcing pilots to turn back and land safely with all 171 passengers and six crew on board. Several passengers suffered injuries. The plane had been in service for jus…