Add visibility in to arc_read

This change is an attempt to add visibility into the arc_read calls
occurring on a system, in real time. To do this, a list was added to the
in memory SPA data structure for a pool, with each element on the list
corresponding to a call to arc_read. These entries are then exported
through the kstat interface, which can then be interpreted in userspace.

For each arc_read call, the following information is exported:

 * A unique identifier (uint64_t)
 * The time the entry was added to the list (hrtime_t)
   (*not* wall clock time; relative to the other entries on the list)
 * The objset ID (uint64_t)
 * The object number (uint64_t)
 * The indirection level (uint64_t)
 * The block ID (uint64_t)
 * The name of the function originating the arc_read call (char[24])
 * The arc_flags from the arc_read call (uint32_t)
 * The PID of the reading thread (pid_t)
 * The command or name of thread originating read (char[16])

From this exported information one can see, in real time, exactly what
is being read, what function is generating the read, and whether or not
the read was found to be already cached.

There is still some work to be done, but this should serve as a good
starting point.

Specifically, dbuf_read's are not accounted for in the currently
exported information. Thus, a follow up patch should probably be added
to export these calls that never call into arc_read (they only hit the
dbuf hash table). In addition, it might be nice to create a utility
similar to "arcstat.py" to digest the exported information and display
it in a more readable format. Or perhaps, log the information and allow
for it to be "replayed" at a later time.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/zfs_context.h

@@ -210,6 +210,7 @@ typedef struct kthread {
 } kthread_t;
 
 #define	curthread			zk_thread_current()
+#define	getcomm()			"unknown"
 #define	thread_exit			zk_thread_exit
 #define	thread_create(stk, stksize, func, arg, len, pp, state, pri)	\
 	zk_thread_create(stk, stksize, (thread_func_t)func, arg,	\
@@ -347,6 +348,10 @@ extern kstat_t *kstat_create(char *, int,
     char *, char *, uchar_t, ulong_t, uchar_t);
 extern void kstat_install(kstat_t *);
 extern void kstat_delete(kstat_t *);
+extern void kstat_set_raw_ops(kstat_t *ksp,
+    int (*headers)(char *buf, size_t size),
+    int (*data)(char *buf, size_t size, void *data),
+    void *(*addr)(kstat_t *ksp, loff_t index));
 
 /*
  * Kernel memory