Add arc_lotsfree_percent module parameter

Traditionally, ARC will throttle allocations when system free memory falls below
10% of total system memory. On some systems, it might be desireable to
use a higher percentage, so lets expose this as a tunable.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Use Linux kzalloc()/kfree() for superblocks

We store the bdi in the superblock. This is accessed using virt_to_page
in bit_waitqueue(), which does not work on kernel virtual memory. We
switch to kzalloc()/kfree() to fix this.

Signed-off-by: Richard Yao <ryao@gentoo.org>