Introduce bpf_xdp_adjust_mb_header helper in order to adjust frame
headers moving *offset* bytes from/to the second buffer to/from the
first one.
This helper can be used to move headers when the hw DMA SG is not able
to copy all the headers in the first fragment and split header and data
pages.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
---
 include/uapi/linux/bpf.h       | 25 ++++++++++++----
 net/core/filter.c              | 54 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 26 ++++++++++++----
 3 files changed, 95 insertions(+), 10 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 8dda13880957..c4a6d245619c 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -3571,11 +3571,25 @@ union bpf_attr {
  *		value.
  *
  * long bpf_copy_from_user(void *dst, u32 size, const void *user_ptr)
- * 	Description
- * 		Read *size* bytes from user space address *user_ptr* and store
- * 		the data in *dst*. This is a wrapper of copy_from_user().
- * 	Return
- * 		0 on success, or a negative error in case of failure.
+ *	Description
+ *		Read *size* bytes from user space address *user_ptr* and store
+ *		the data in *dst*. This is a wrapper of copy_from_user().
+ *
+ * long bpf_xdp_adjust_mb_header(struct xdp_buff *xdp_md, int offset)