bpf: Clarify the meaning of BPF_F_PSEUDO_HDR

In the bpf_l4_csum_replace helper, the BPF_F_PSEUDO_HDR flag should only
be set if the modified header field is part of the pseudo-header.

If you modify for example the UDP ports and pass BPF_F_PSEUDO_HDR,
inet_proto_csum_replace4 will update skb->csum even though it shouldn't
(the port and the UDP checksum updates null each other).

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/5126ef84ba75425b689482cbc98bffe75e5d8ab0.1744102490.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: pchaigno

include/uapi/linux/bpf.h

@@ -2055,7 +2055,7 @@ union bpf_attr {
  * 		untouched (unless **BPF_F_MARK_ENFORCE** is added as well), and
  * 		for updates resulting in a null checksum the value is set to
  * 		**CSUM_MANGLED_0** instead. Flag **BPF_F_PSEUDO_HDR** indicates
- * 		the checksum is to be computed against a pseudo-header.
+ * 		that the modified header field is part of the pseudo-header.
  *
  * 		This helper works in combination with **bpf_csum_diff**\ (),
  * 		which does not update the checksum in-place, but offers more

tools/include/uapi/linux/bpf.h

@@ -2055,7 +2055,7 @@ union bpf_attr {
  * 		untouched (unless **BPF_F_MARK_ENFORCE** is added as well), and
  * 		for updates resulting in a null checksum the value is set to
  * 		**CSUM_MANGLED_0** instead. Flag **BPF_F_PSEUDO_HDR** indicates
- * 		the checksum is to be computed against a pseudo-header.
+ * 		that the modified header field is part of the pseudo-header.
  *
  * 		This helper works in combination with **bpf_csum_diff**\ (),
  * 		which does not update the checksum in-place, but offers more