Change os->os_next_write_raw to work per txg

Currently, os_next_write_raw is a single boolean used for determining
whether or not the next call to dmu_objset_sync() should write out
the objset_phys_t as a raw buffer. Since the boolean is not associated
with a txg, the work simply happens during the next txg, which is not
necessarily the correct one. In the current implementation this issue
was misdiagnosed, resulting in a small hack in dmu_objset_sync() which
seemed to resolve the problem.

This patch changes os_next_write_raw to be an array of booleans, one
for each txg in TXG_OFF and removes the hack.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #6864

include/sys/dmu_objset.h

@@ -127,7 +127,7 @@ struct objset {
 	boolean_t os_rescan_dnodes;
 
 	/* os_phys_buf should be written raw next txg */
-	boolean_t os_next_write_raw;
+	boolean_t os_next_write_raw[TXG_SIZE];
 
 	/* Protected by os_obj_lock */
 	kmutex_t os_obj_lock;