net: allow alloc_skb_with_frags() to use MAX_SKB_FRAGS

Currently, alloc_skb_with_frags() will only fill (MAX_SKB_FRAGS - 1)
slots. I think it should use all MAX_SKB_FRAGS slots, as callers of
alloc_skb_with_frags() will size their allocation of frags based
on MAX_SKB_FRAGS.

This issue was discovered via a test patch that sets 'order' to 0
in alloc_skb_with_frags(), which effectively tests/simulates high
fragmentation. In this case sendmsg() on unix sockets will fail every
time for large allocations. If the PAGE_SIZE is 4K, then data_len will
request 68K or 17 pages, but alloc_skb_with_frags() can only allocate
64K in this case or 16 pages.

Fixes: 09c2c90 ("net: allow alloc_skb_with_frags() to allocate bigger packets")
Signed-off-by: Jason Baron <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: jbaron-lgtm

net/core/skbuff.c

@@ -6667,7 +6667,7 @@ struct sk_buff *alloc_skb_with_frags(unsigned long header_len,
 		return NULL;
 
 	while (data_len) {
-		if (nr_frags == MAX_SKB_FRAGS - 1)
+		if (nr_frags == MAX_SKB_FRAGS)
 			goto failure;
 		while (order && PAGE_ALIGN(data_len) < (PAGE_SIZE << order))
 			order--;