Revert "Add zio constructor/destructor"

This patch was slightly flawed and allowed for zio->io_logical
to potentially not be reinitialized for a new zio.  This could
lead to assertion failures in specific cases when debugging is
enabled (--enable-debug) and I/O errors are encountered.  It
may also have caused problems when issues logical I/Os.

Since we want to make sure this workaround can be easily removed
in the future (when we have the real fix).  I'm reverting this
change and applying a new version of the patch which includes
the zio->io_logical fix.

This reverts commit 2c6d0b1e07b0265f0661ed7851d3aa8d3e75e7a9.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #602
Issue #604
Add zio constructor/destructor

Add a standard zio constructor and destructor.  Normally, this is
done to reduce to cost of allocating a new structure by reducing
expensive operations such as memory allocations.  However, in this
case none of the operations moved out of zio_create() were really
very expensive.

This change was principly made as a debug patch (and workaround)
for a zio_destroy() race.  The is good evidence that zio_create()
is reinitializing a mutex which is really still in use by another
thread.  This would completely explain the observed symptoms in
the issue report.

This patch doesn't fix the root cause of the race, but it should
make it less likely by only initializing the mutex once in the
constructor.  Also, this particular flaw might have gone unnoticed
in other zfs implementations due to the specific implementation
details of Linux ticket spinlocks.

Once the real root cause is determined and resolved this change
can be safely reverted.  Until then this should help workaround
the issue.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #496
Add .zfs control directory

Add support for the .zfs control directory.  This was accomplished
by leveraging as much of the existing ZFS infrastructure as posible
and updating it for Linux as required.  The bulk of the core
functionality is now all there with the following limitations.

*) The .zfs/snapshot directory automount support requires a 2.6.37
   or newer kernel.  The exception is RHEL6.2 which has backported
   the d_automount patches.

*) Creating/destroying/renaming snapshots with mkdir/rmdir/mv
   in the .zfs/snapshot directory works as expected.  However,
   this functionality is only available to root until zfs
   delegations are finished.

      * mkdir - create a snapshot
      * rmdir - destroy a snapshot
      * mv    - rename a snapshot

The following issues are known defeciences, but we expect them to
be addressed by future commits.

*) Add automount support for kernels older the 2.6.37.  This should
   be possible using follow_link() which is what Linux did before.

*) Accessing the .zfs/snapshot directory via NFS is not yet possible.
   The majority of the ground work for this is complete.  However,
   finishing this work will require resolving some lingering
   integration issues with the Linux NFS kernel server.

*) The .zfs/shares directory exists but no futher smb functionality
   has yet been implemented.

Contributions-by: Rohan Puri <rohan.puri15@gmail.com>
Contributiobs-by: Andrew Barnes <barnes333@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #173
Account for .zfs ctldir inodes

Because the .zfs ctldir inodes are not backed by physical storage
they use a different create path which was not properly accounting
for them as used.  This could result in ->nr_cached_objects()
returning 0 and cause a divide by zero error in prune_super().

In my option there's a kernel bug here too which allows this to
happen.  They should either be checking for 0 or adding +1 like
they correctly do earlier in the function.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #617
Merge branch 'master' of git://github.com/zfsonlinux/zfs