docs: kernel_feat.py: fix build error for missing files

If the directory passed to the '.. kernel-feat::' directive does not
exist or the get_feat.pl script does not find any files to extract
features from, Sphinx will report the following error:

    Sphinx parallel build error:
    UnboundLocalError: local variable 'fname' referenced before assignment
    make[2]: *** [Documentation/Makefile:102: htmldocs] Error 2

This is due to how I changed the script in c48a7c44 ("docs:
kernel_feat.py: fix potential command injection"). Before that, the
filename passed along to self.nestedParse() in this case was weirdly
just the whole get_feat.pl invocation.

We can fix it by doing what kernel_abi.py does -- just pass
self.arguments[0] as 'fname'.

Fixes: c48a7c44

 ("docs: kernel_feat.py: fix potential command injection")
Cc: Justin Forbes <jforbes@fedoraproject.org>
Cc: Salvatore Bonaccorso <carnil@debian.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Link: https://lore.kernel.org/r/20240205175133.774271-2-vegard.nossum@oracle.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>