Fix range locking in ZIL commit codepath

Since OpenZFS 7578 (1b7c1e5) if we have a ZVOL with logbias=throughput
we will force WR_INDIRECT itxs in zvol_log_write() setting itx->itx_lr
offset and length to the offset and length of the BIO from
zvol_write()->zvol_log_write(): these offset and length are later used
to take a range lock in zillog->zl_get_data function: zvol_get_data().

Now suppose we have a ZVOL with blocksize=8K and push 4K writes to
offset 0: we will only be range-locking 0-4096. This means the
ASSERTion we make in dbuf_unoverride() is no longer valid because now
dmu_sync() is called from zilog's get_data functions holding a partial
lock on the dbuf.

Fix this by taking a range lock on the whole block in zvol_get_data().

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6238 
Closes #6315 
Closes #6356 
Closes #6477

module/zfs/dmu.c

@@ -49,6 +49,7 @@
 #include <sys/zfeature.h>
 #include <sys/abd.h>
 #include <sys/trace_dmu.h>
+#include <sys/zfs_rlock.h>
 #ifdef _KERNEL
 #include <sys/vmsystm.h>
 #include <sys/zfs_znode.h>
@@ -1815,6 +1816,11 @@ dmu_sync(zio_t *pio, uint64_t txg, dmu_sync_cb_t *done, zgd_t *zgd)
 	ASSERT(pio != NULL);
 	ASSERT(txg != 0);
 
+	/* dbuf is within the locked range */
+	ASSERT3U(db->db.db_offset, >=, zgd->zgd_rl->r_off);
+	ASSERT3U(db->db.db_offset + db->db.db_size, <=,
+	    zgd->zgd_rl->r_off + zgd->zgd_rl->r_len);
+
 	SET_BOOKMARK(&zb, ds->ds_object,
 	    db->db.db_object, db->db_level, db->db_blkid);