Add zfs_sync_pass_* tunable parameters

Commit 55d85d5 (backport of
the upstream changes) replaced three hardcoded constants:

    #define SYNC_PASS_DEFERRED_FREE 2 /* defer frees after this pass */
    #define SYNC_PASS_DONT_COMPRESS 4 /* don't compress after this pass */
    #define SYNC_PASS_REWRITE       1 /* rewrite new bps after this pass */

with a tunable parameters:

    int zfs_sync_pass_deferred_free = 2; /* defer frees starting in this pass */
    int zfs_sync_pass_dont_compress = 5; /* don't compress starting in this pass */
    int zfs_sync_pass_rewrite = 2;       /* rewrite new bps starting in this pass */

This commit makes these tunables available as module parameters
in Linux.  They should only be used for performance analysis
because changing them can result in subtle and pathological
performance problems.

Signed-off-by: Brian Behlendorf <[email protected]>
Closes #1562

module/zfs/zio.c

@@ -3236,4 +3236,16 @@ MODULE_PARM_DESC(zio_delay_max, "Max zio millisec delay before posting event");
 
 module_param(zio_requeue_io_start_cut_in_line, int, 0644);
 MODULE_PARM_DESC(zio_requeue_io_start_cut_in_line, "Prioritize requeued I/O");
+
+module_param(zfs_sync_pass_deferred_free, int, 0644);
+MODULE_PARM_DESC(zfs_sync_pass_deferred_free,
+    "defer frees starting in this pass");
+
+module_param(zfs_sync_pass_dont_compress, int, 0644);
+MODULE_PARM_DESC(zfs_sync_pass_dont_compress,
+    "don't compress starting in this pass");
+
+module_param(zfs_sync_pass_rewrite, int, 0644);
+MODULE_PARM_DESC(zfs_sync_pass_rewrite,
+    "rewrite new bps starting in this pass");
 #endif