From: David Gibson <david@gibson.dropbear.id.au>

The patch below fixes a bug in ppc64's 32-bit execve() path.  It duplicates
logic already in the normal fs/exec.c do_execve() to avoid dropping a NULL
mm.  The bprm.mm becomes NULL once the exec passes the "point of no
return".  Without this patch a failure past that point (e.g.  mmap()
failure) will cause an oops, with it just a killed process.