pipe: Verify readopen before locking
pipe: Verify readopen before locking

Fixes sharing between writes to closed pipes.

aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6