LU-6142 gss: SPDX for GSS

LU-6142 gss: SPDX for GSS

Convert from verbose license text to SPDX. These files are
largely derived from in-kernel sunrpc GSS code, which derived
it from the Kerberos project.

I've tracked down each file in upstream Linux - and either
applied the correct license to the Lustre version or omitted
the SPDX (as the kernel does) along with an updated full-path
to the Linux kernel file.

Also, add the BSD-3-Clause license text.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I19283cb8d3d625842984b9112014cc58a5a04726
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57822
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>