ext4: use journal inode to determine journal overhead

When a file system contains an internal journal that has not been
loaded, use the journal inode's i_size field to determine its
contribution to the file system's overhead.  (The journal's j_maxlen
field is normally used to determine its size, but it's unavailable when
the journal has not been loaded.)

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>