Add comments for portable dnode / objset flags

This patch adds some comments describing the purpose of "portable"
dnode and objset flags so that it is clear when new flags should
be added to the repective flag masks. This patch includes no
functional changes.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7313

include/sys/dnode.h

@@ -150,6 +150,12 @@ enum dnode_dirtycontext {
 /* User/Group/Project dnode accounting */
 #define	DNODE_FLAG_USEROBJUSED_ACCOUNTED	(1 << 3)
 
+/*
+ * This mask defines the set of flags which are "portable", meaning
+ * that they can be preserved when doing a raw encrypted zfs send.
+ * Flags included in this mask will be protected by AAD when the block
+ * of dnodes is encrypted.
+ */
 #define	DNODE_CRYPT_PORTABLE_FLAGS_MASK		(DNODE_FLAG_SPILL_BLKPTR)
 
 /*