xen: check if there are clients waiting in gnttab_end_foreign_access_references

Without a call to check_free_callbacks() clients waiting for grant
references would not be woken up even when there are sufficient grant
references available.

The check was likely left out as a mistake when the function was first
added.

Note that other functions used to free grant references already call
check_free_callbacks.

Submitted by:		pratyush
Reviewed by:		royger
Differential review:	https://reviews.freebsd.org/D15899

sys/dev/xen/grant_table/grant_table.c

@@ -245,6 +245,7 @@ gnttab_end_foreign_access_references(u_int count, grant_ref_t *refs)
 		gnttab_free_count += count;
 		gnttab_entry(tail) = gnttab_free_head;
 		gnttab_free_head = head;
+		check_free_callbacks();
 		mtx_unlock(&gnttab_list_lock);
 	}
 }