Make kstat.ks_update() callback atomic

Move the kstat ks_update() callback under the ks_lock.  This
enables dynamically sized kstats without modification to the
kstat API.

  * Create a kstat with the KSTAT_FLAG_VIRTUAL flag.
  * Register a ->ks_update() callback which does:
    o Frees any existing ks_data buffer.
    o Set ks_data_size to the kstat array size.
    o Set ks_data to an allocated buffer of size ks_data_size
    o Populate the array of buffers with the required data.

The buffer allocated in the ks_update() callback is guaranteed
to remain allocated and valid while the proc sequence handler
iterates over the buffer.  The lock will not be dropped until
kstat_seq_stop() function is run making it safe for concurrent
access.  To allow the ks_update() callback to perform memory
allocations the lock was changed to a mutex.

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

include/sys/kstat.h

@@ -30,6 +30,7 @@
 #include <sys/types.h>
 #include <sys/time.h>
 #include <sys/kmem.h>
+#include <sys/mutex.h>
 
 #define KSTAT_STRLEN            31
 
@@ -98,7 +99,7 @@ typedef struct kstat_s {
         struct proc_dir_entry *ks_proc;             /* proc linkage */
         kstat_update_t   *ks_update;                /* dynamic updates */
         void             *ks_private;               /* private data */
-        spinlock_t       ks_lock;                   /* kstat data lock */
+        kmutex_t         ks_lock;                   /* kstat data lock */
         struct list_head ks_list;                   /* kstat linkage */
 } kstat_t;