bpf: Fix documentation of th_len in bpf_tcp_{gen,check}_syncookie

bpf_tcp_gen_syncookie expects the full length of the TCP header (with
all options), and bpf_tcp_check_syncookie accepts lengths bigger than
sizeof(struct tcphdr). Fix the documentation that says these lengths
should be exactly sizeof(struct tcphdr).

While at it, fix a typo in the name of struct ipv6hdr.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20220615134847.3753567-2-maximmi@nvidia.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -3597,10 +3597,11 @@ union bpf_attr {
  *
  * 		*iph* points to the start of the IPv4 or IPv6 header, while
  * 		*iph_len* contains **sizeof**\ (**struct iphdr**) or
- * 		**sizeof**\ (**struct ip6hdr**).
+ * 		**sizeof**\ (**struct ipv6hdr**).
  *
  * 		*th* points to the start of the TCP header, while *th_len*
- * 		contains **sizeof**\ (**struct tcphdr**).
+ *		contains the length of the TCP header (at least
+ *		**sizeof**\ (**struct tcphdr**)).
  * 	Return
  * 		0 if *iph* and *th* are a valid SYN cookie ACK, or a negative
  * 		error otherwise.
@@ -3783,10 +3784,11 @@ union bpf_attr {
  *
  *		*iph* points to the start of the IPv4 or IPv6 header, while
  *		*iph_len* contains **sizeof**\ (**struct iphdr**) or
- *		**sizeof**\ (**struct ip6hdr**).
+ *		**sizeof**\ (**struct ipv6hdr**).
  *
  *		*th* points to the start of the TCP header, while *th_len*
- *		contains the length of the TCP header.
+ *		contains the length of the TCP header with options (at least
+ *		**sizeof**\ (**struct tcphdr**)).
  *	Return
  *		On success, lower 32 bits hold the generated SYN cookie in
  *		followed by 16 bits which hold the MSS value for that cookie,

tools/include/uapi/linux/bpf.h

@@ -3597,10 +3597,11 @@ union bpf_attr {
  *
  * 		*iph* points to the start of the IPv4 or IPv6 header, while
  * 		*iph_len* contains **sizeof**\ (**struct iphdr**) or
- * 		**sizeof**\ (**struct ip6hdr**).
+ * 		**sizeof**\ (**struct ipv6hdr**).
  *
  * 		*th* points to the start of the TCP header, while *th_len*
- * 		contains **sizeof**\ (**struct tcphdr**).
+ *		contains the length of the TCP header (at least
+ *		**sizeof**\ (**struct tcphdr**)).
  * 	Return
  * 		0 if *iph* and *th* are a valid SYN cookie ACK, or a negative
  * 		error otherwise.
@@ -3783,10 +3784,11 @@ union bpf_attr {
  *
  *		*iph* points to the start of the IPv4 or IPv6 header, while
  *		*iph_len* contains **sizeof**\ (**struct iphdr**) or
- *		**sizeof**\ (**struct ip6hdr**).
+ *		**sizeof**\ (**struct ipv6hdr**).
  *
  *		*th* points to the start of the TCP header, while *th_len*
- *		contains the length of the TCP header.
+ *		contains the length of the TCP header with options (at least
+ *		**sizeof**\ (**struct tcphdr**)).
  *	Return
  *		On success, lower 32 bits hold the generated SYN cookie in
  *		followed by 16 bits which hold the MSS value for that cookie,