mm/vmalloc: fix return value of vb_alloc if size is 0

vm_map_ram() uses IS_ERR() to validate the return value of vb_alloc().  If
vm_map_ram(page, 0, 0) is executed, vb_alloc(0, GFP_KERNEL) would return
NULL.  In such a case, IS_ERR() cannot handle the return value and lead to
kernel panic by vmap_pages_range_noflush() at last.  To resolve this
issue, return ERR_PTR(-EINVAL) if the size is 0.

Link: https://lkml.kernel.org/r/[email protected]
Reviewed-by: Barry Song <[email protected]>
Reviewed-by: Uladzislau Rezki (Sony) <[email protected]>
Signed-off-by: Hailong.Liu <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: hailong-liu

mm/vmalloc.c

@@ -2710,7 +2710,7 @@ static void *vb_alloc(unsigned long size, gfp_t gfp_mask)
 		 * get_order(0) returns funny result. Just warn and terminate
 		 * early.
 		 */
-		return NULL;
+		return ERR_PTR(-EINVAL);
 	}
 	order = get_order(size);