Increase ZFS_OBJ_MTX_SZ further

Larger values have been associated with better concurrency and less
hanging of memory reclaim operations. So raise this number
further.
Add option to zpool status to print guids

The use of vdev GUIDs are a necessary workaround in edge cases where the
names provided by `zpool status` are not accepted by the zpool
detach/offline/remove/replace commands. The current method of obtaining
them uses zdb, but this does not work in all cases (see
zfsonlinux/zfs#1530).

This provides a method of obtaining vdev GUIDs that is more reliable and
straightforward than zdb. It would be better to fix all edge cases that
require the use of GUIDs as a workaround, but Linux's /dev design makes
it difficult to anticipate such edge cases, which makes this option
necessary.

Note that this adds a new boolean parameter to `zpool_vdev_name`, which
changes the libzfs interface.

Closes zfsonlinux/zfs#2011

Signed-off-by: Richard Yao <ryao@gentoo.org>
Add "zpool nagios" command

(Though it could use improvement, this feature is working.)

The "zpool nagios" command behaves as a Nagios plugin - output a
single line of status information to stdout and exit with a status
code (0=OK, 1=Warning, 2=Critical, 3=Unknown).

There are a few cases that could use improvement for multi-pool
systems, and not all failure scenarios are tested, but it's
certainly a step in the right direction.

TODO:
* Specify pools which must be present on the system
* Additional testing. I don't have a lot of broken pools to mess with.
* Deadman timer of sorts in case ioctls hang
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>