Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored.  This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required, the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing.  This ensures that a clean dnode can't be dirtied before
the data/hole is located.  The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12724

module/zfs/dnode.c

@@ -1648,6 +1648,26 @@ dnode_try_claim(objset_t *os, uint64_t object, int slots)
 	    slots, NULL, NULL));
 }
 
+/*
+ * Checks if the dnode contains any uncommitted dirty records.
+ */
+boolean_t
+dnode_is_dirty(dnode_t *dn)
+{
+	mutex_enter(&dn->dn_mtx);
+
+	for (int i = 0; i < TXG_SIZE; i++) {
+		if (list_head(&dn->dn_dirty_records[i]) != NULL) {
+			mutex_exit(&dn->dn_mtx);
+			return (B_TRUE);
+		}
+	}
+
+	mutex_exit(&dn->dn_mtx);
+
+	return (B_FALSE);
+}
+
 void
 dnode_setdirty(dnode_t *dn, dmu_tx_t *tx)
 {