binfmt_elf.c: use get_random_int() to fix entropy depleting.

Usually the entropy pool depletes very fast and stays in the 100-200 range.
After this patch, the entropy stays in the 600-1000 range without the risk of
depleting in normal workloads.

Change-Id: Ice0d0f14691046249d7b4a4d31af98ab80c6306b