Add sa_spill_rele() interface

Add a SA interface which allows us to release the spill block
from a SA handle without destroying the handle.  This is useful
because we can then ensure that a copy of the dirty spill block
is not made at sync time due to the extra hold.  Susequent calls
to sa_update() or sa_lookup() with transparently refetch the
spill block dbuf from the ARC hash.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/sa.h

@@ -139,6 +139,7 @@ void sa_set_userp(sa_handle_t *, void *);
 dmu_buf_t *sa_get_db(sa_handle_t *);
 uint64_t sa_handle_object(sa_handle_t *);
 boolean_t sa_attr_would_spill(sa_handle_t *, sa_attr_type_t, int size);
+void sa_spill_rele(sa_handle_t *);
 void sa_register_update_callback(objset_t *, sa_update_cb_t *);
 int sa_setup(objset_t *, uint64_t, sa_attr_reg_t *, int, sa_attr_type_t **);
 void sa_tear_down(objset_t *);