kbuild: deb-pkg: change the source package name to linux-upstream

kbuild: deb-pkg: change the source package name to linux-upstream

Change the source package name from 'linux-$(KERNELRELEASE)' to
'linux-upstream'.

Initially, I tried to use 'linux' to be aligned with the Debian
kernel package, but Ben suggested 'linux-upstream' so that it is
clearly distinguished from distribution packages. [1]

The filenames will be changed as follows:

[Before]
  linux-5.12.0-rc3+_5.12.0-rc3+-1.dsc
  linux-5.12.0-rc3+_5.12.0-rc3+.orig.tar.gz
  linux-5.12.0-rc3+_5.12.0-rc3+-1.diff.gz

[After]
  linux-upstream_5.12.0-rc3+-1.dsc
  linux-upstream_5.12.0-rc3+.orig.tar.gz
  linux-upstream_5.12.0-rc3+-1.diff.gz

Commit 3716001bcb7f ("deb-pkg: add source package") introduced
KDEB_SOURCENAME. If you are unhappy with the default name, you can
override it via KDEB_SOURCENAME.

[1]: https://lore.kernel.org/linux-kbuild/06ffa2a690d57f867b4bc1b42f0026917b1dd3cd.camel@decadent.org.uk/T/#m2c4afa0eca5ced5e57795b002f2dbcb05d7a4a44

Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>