Retire legacy debugging infrastructure

When the SPL was originally written Linux tracepoints were still
in their infancy.  Therefore, an entire debugging subsystem was
added to facilite tracing which served us well for many years.

Now that Linux tracepoints have matured they provide all the
functionality of the previous tracing subsystem.  Rather than
maintain parallel functionality it makes sense to fully adopt
tracepoints.  Therefore, this patch retires the legacy debugging
infrastructure.

See zfsonlinux/zfs@bc9f413 for the tracepoint changes.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #408
Remove compat includes from sys/types.h

Don't include the compatibility code in linux/*_compat.h in the public
header sys/types.h. This causes problems when an external code base
includes the ZFS headers and has its own conflicting compatibility code.
Lustre, in particular, defined SHRINK_STOP for compatibility with
pre-3.12 kernels in a way that conflicted with the SPL's definition.
Because Lustre ZFS OSD includes ZFS headers it fails to build due to a
'"SHRINK_STOP" redefined' compiler warning.  To avoid such conflicts
only include the compat headers from .c files or private headers.

Also, for consistency, include sys/*.h before linux/*.h then sort by
header name.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #411
mutex: force serialization on mutex_exit() to fix races

It is known that mutexes in Linux are not safe when using them to
synchronize the freeing of object in which the mutex is embedded:

http://lwn.net/Articles/575477/

The known places in ZFS which are suspected to suffer from the race
condition are zio->io_lock and dbuf->db_mtx.

* zio uses zio->io_lock and zio->io_cv to synchronize freeing
  between zio_wait() and zio_done().
* dbuf uses dbuf->db_mtx to protect reference counting.

This patch fixes this kind of race by forcing serialization on
mutex_exit() with a spin lock, making the mutex safe by sacrificing
a bit of performance and memory overhead.

This issue most commonly manifests itself as a deadlock in the zio
pipeline caused by a process spinning on the damaged mutex.  Similar
deadlocks have been reported for the dbuf->db_mtx mutex.  And it can
also cause a NULL dereference or bad paging request under the right
circumstances.

This issue any many like it are linked off the zfsonlinux/zfs#2523
issue.  Specifically this fix resolves at least the following
outstanding issues:

zfsonlinux/zfs#401
zfsonlinux/zfs#2523
zfsonlinux/zfs#2679
zfsonlinux/zfs#2684
zfsonlinux/zfs#2704
zfsonlinux/zfs#2708
zfsonlinux/zfs#2517
zfsonlinux/zfs#2827
zfsonlinux/zfs#2850
zfsonlinux/zfs#2891
zfsonlinux/zfs#2897
zfsonlinux/zfs#2247
zfsonlinux/zfs#2939

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #421