gnttab: remove pointless NULL check

Domains in the domain hash (and hence locatable via the usual lookup
functions) can't have a NULL grant table pointer; no other function
performs such a check, so remove it from gnttab_prepare_for_transfer()
for consistency.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1390,17 +1390,11 @@ static int 
 gnttab_prepare_for_transfer(
     struct domain *rd, struct domain *ld, grant_ref_t ref)
 {
-    struct grant_table *rgt;
+    struct grant_table *rgt = rd->grant_table;
     grant_entry_header_t *sha;
     union grant_combo   scombo, prev_scombo, new_scombo;
     int                 retries = 0;
 
-    if ( unlikely((rgt = rd->grant_table) == NULL) )
-    {
-        gdprintk(XENLOG_INFO, "Dom %d has no grant table.\n", rd->domain_id);
-        return 0;
-    }
-
     spin_lock(&rgt->lock);
 
     if ( rgt->gt_version == 0 )