Yankee Al nailed it. This is a classic play to illustrate the importance of WHEN in the 4 W's of penalty enforcement. The calling official needs to recognize the time of the foul, not just the foul itself.
IF....
(1) The foul occurred prior to the interception = loose ball, possible DPI, previous spot;
(2) after the pick, but prior to/during the fumble = end of related run, touchback, all-but-one;
(3) after the fumble recovery but before ball became dead = if ball stayed @ B's 4, end of that related run - ball placed @ B's 2;
(4) after ball became dead = waive off flag, no dead-ball holding.
....NOW , WHAT IF THE HOLDING OCCURRED IN B'S ENDZONE
??