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.