Fix function call with uninitialized value in vdev_inuse

LLVM's static analyzer reported that we could pass an uninitialized
pool_guid to spa_by_guid() in vdev_inuse(). Upon review, it is correct.
An attempt to repurpose a spare or L2ARC drive from an exported pool
will cause the pool_guid passed to spa_by_guid() to be unintialized
information from the stack. This will cause non-deterministic behavior.
Since there is no reason why we cannot repurpose such disks, we modify
vdev_inuse() to avoid calling spa_by_guid() when they are detected.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Fix memory leak in zpool_clear_label()

Clang's static analyzer reported a memory leak in zpool_clear_label().
Upon review, it turns out to be right. This should be a very short lived
leak because no daemons use this functionality, but that does not
preclude the possibility of third party daemons that do use it. Lets fix
it to be a good Samaritan.

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

Clang's static analyzer reported that the value assigned to pcksum is
never used. That is because we initialize both zc and pcksum to {{ 0 }}
and then do `pcksum = zc;`. That is fairly pointless. However, it has
the effect of generating a false positive in Clang's static analyzer.
Since noise from false positives can obscure real issues, we fix it
anyway.

Signed-off-by: Richard Yao <ryao@gentoo.org>
lib/libzpool/kernel.c: Assert no owners in rw_destroy()

This is intended to cause ztest to fail when rw_destroy() is called on a
rwlock that has owners.

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