Add callbacks for displaying KSTAT_TYPE_RAW kstats

The current implementation for displaying kstats of type KSTAT_TYPE_RAW
is rather crude. This patch attempts to enhance this handling by
allowing a kstat user to register formatting callbacks which can
optionally be used.

The callbacks allow the user to implement functions for interpreting
their data and transposing it into a character buffer. This buffer,
containing a string representation of the raw data, is then be displayed
through the current /proc textual interface.

Additionally the kstats are made writable because it's now possible
to provide a useful handler via the existing ks_update() interface.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #296
Add wrappers for accessing PID and command info

This change adds simple wrappers for accessing a thread's PID and
command character string.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #296
Revert "Add KSTAT_TYPE_TXG type"

This reverts commit dba79fcbf2cc50be5caef84ae01657e884ac5d89 in
favor of using the generic KSTAT_TYPE_RAW callbacks.  The advantage
of this approach is that arbitrary types can be added without the
need to add them to the SPL.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #296
kstat to use private lock by default

While porting Illumos #3537 I found that ks_lock member of kstat_t
structure is different between Illumos and SPL. It is a pointer to
the kmutex_t in Illumos, but the mutex lock itself in SPL.
Apparently Illumos kstat API allows consumer to override the lock
if required. With SPL implementation it is not possible anymore.

Things were alright until the first attempt to actually override
the lock. Porting of Illumos #3537 introduced such code for the
first time.

In order to provide the Solaris/Illumos like functionality we:
  1. convert ks_lock to "kmutex_t *ks_lock"
  2. create a new field "kmutex_t ks_private_lock"
  3. On kstat_create() ks_lock = &ks_private_lock

Thus if consumer doesn't care we still have our internal lock in use.
If, however, consumer does care she has a chance to set ks_lock to
anything else before calling kstat_install().

The rest of the code will use ks_lock regardless of its origin.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #286
3537 add kstat_waitq_enter and friends

These kstat interfaces are required to port
"Illumos #3537 want pool io kstats" to ZFS on Linux.

kstat_waitq_enter()
kstat_waitq_exit()
kstat_runq_enter()
kstat_runq_exit()

Additionally, zero out the ks_data buffer in __kstat_create() so
that the kstat_io_t counters are initialized to zero.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>