vma: Don't lookup file_desc for vma twice

We do it first -- on collect, second -- on restore. The
2nd lookup is excessive, we can put fd pointer on vm_area
at lookup and reuse one later.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files.c

@@ -1087,11 +1087,6 @@ out:
 	return ret;
 }
 
-int get_filemap_fd(int pid, VmaEntry *vma_entry)
-{
-	return open_reg_by_id(vma_entry->shmid);
-}
-
 int shared_fdt_prepare(struct pstree_item *item)
 {
 	struct pstree_item *parent = item->parent;