Revert "writeback: increase bdi_min_ratio to 5 in light of the latest writeback commit."

This reverts commit 94370584d28ec5240f55d20421dd2118e5da5b3d.
Revert "binfmt_elf.c: use get_random_int() to fix entropy depleting"

This reverts commit 06d1bf53706d28c959b52d3cec25fba77b058844.