dmu_zfetch: fix memory leak

The last change caused the read completion callback to not be called
if the IO was still in progress. This change restores allocation
of the arc buf callback, but in the callback path checks the new
acb_nobuf field to know to skip buffer allocation.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11324

include/sys/arc_impl.h

@@ -99,6 +99,7 @@ struct arc_callback {
 	boolean_t		acb_encrypted;
 	boolean_t		acb_compressed;
 	boolean_t		acb_noauth;
+	boolean_t		acb_nobuf;
 	zbookmark_phys_t	acb_zb;
 	zio_t			*acb_zio_dummy;
 	zio_t			*acb_zio_head;