docs: generate a separate ext4 pdf file from the documentation

The documentation build scripts won't build a pdf for the ext4
documentation unless explicitly called for, so ask for a separate
ext4.pdf to be generated with all the documentation.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

Documentation/conf.py

@@ -383,6 +383,8 @@ latex_documents = [
      'The kernel development community', 'manual'),
     ('filesystems/index', 'filesystems.tex', 'Linux Filesystems API',
      'The kernel development community', 'manual'),
+    ('filesystems/ext4/index', 'ext4.tex', 'ext4 Filesystem',
+     'ext4 Filesystem Developers', 'manual'),
     ('gpu/index', 'gpu.tex', 'Linux GPU Driver Developer\'s Guide',
      'The kernel development community', 'manual'),
     ('input/index', 'linux-input.tex', 'The Linux input driver subsystem',