bpf, docs: Use SPDX license identifier in bpf_doc.py

The Linux man-pages project now uses SPDX tags, instead of the full
license text.
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://www.kernel.org/doc/man-pages/licenses.html
Link: https://spdx.org/licenses/Linux-man-pages-copyleft.html
Link: https://lore.kernel.org/bpf/20220721110821.8240-1-alx.manpages@gmail.com

scripts/bpf_doc.py

@@ -333,27 +333,7 @@ class PrinterRST(Printer):
 .. Copyright (C) All BPF authors and contributors from 2014 to present.
 .. See git log include/uapi/linux/bpf.h in kernel tree for details.
 .. 
-.. %%%LICENSE_START(VERBATIM)
-.. Permission is granted to make and distribute verbatim copies of this
-.. manual provided the copyright notice and this permission notice are
-.. preserved on all copies.
-.. 
-.. Permission is granted to copy and distribute modified versions of this
-.. manual under the conditions for verbatim copying, provided that the
-.. entire resulting derived work is distributed under the terms of a
-.. permission notice identical to this one.
-.. 
-.. Since the Linux kernel and libraries are constantly changing, this
-.. manual page may be incorrect or out-of-date.  The author(s) assume no
-.. responsibility for errors or omissions, or for damages resulting from
-.. the use of the information contained herein.  The author(s) may not
-.. have taken the same level of care in the production of this manual,
-.. which is licensed free of charge, as they might when working
-.. professionally.
-.. 
-.. Formatted or processed versions of this manual, if unaccompanied by
-.. the source, must acknowledge the copyright and authors of this work.
-.. %%%LICENSE_END
+.. SPDX-License-Identifier:  Linux-man-pages-copyleft
 .. 
 .. Please do not edit this file. It was generated from the documentation
 .. located in file include/uapi/linux/bpf.h of the Linux kernel sources