assertion in arc_release() during encrypted receive

In the existing code, when doing a raw (encrypted) zfs receive, 
we call arc_convert_to_raw() from open context. This creates a 
race condition between arc_release()/arc_change_state() and 
writing out the block from syncing context (arc_write_ready/done()).

This change makes it so that when we are doing a raw (encrypted) 
zfs receive, we save the crypt parameters (salt, iv, mac) of dnode 
blocks in the dbuf_dirty_record_t, and call arc_convert_to_raw() 
from syncing context when writing out the block of dnodes.

Additionally, we can eliminate dr_raw and associated setters, and 
instead know that dnode blocks are always raw when doing a zfs 
receive (see the new field os_raw_receive).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7424 
Closes #7429

include/sys/dmu_objset.h

@@ -139,6 +139,7 @@ struct objset {
 	uint64_t os_flags;
 	uint64_t os_freed_dnodes;
 	boolean_t os_rescan_dnodes;
+	boolean_t os_raw_receive;
 
 	/* os_phys_buf should be written raw next txg */
 	boolean_t os_next_write_raw[TXG_SIZE];