selftests: Keep symlinks, when possible

When kselftest is built/installed with the 'gen_tar' target, rsync is
used for the installation step to copy files. Extra care is needed for
tests that have symlinks. Commit ae108c48 ("selftests: net: Fix
cross-tree inclusion of scripts") added '-L' (transform symlink into
referent file/dir) to rsync, to fix dangling links. However, that
broke some tests where the symlink (being a symlink) is part of the
test (e.g. exec:execveat).

Use rsync's '--copy-unsafe-links' that does right thing.

Fixes: ae108c48

 ("selftests: net: Fix cross-tree inclusion of scripts")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>