df: reorder default field list of --output option

df: reorder default field list of --output option

As the inodes information is usually not so much of interest,
and some file systems including btrfs do not even provide it,
reading of the full df --output is easier when the block
statistic fields come just left of the last field, the mount
point.

* src/df.c (all_args_string): Move the inodes fields before
the block fields.
(usage): Likewise.
* tests/df/df-output.sh: Likewise.
* doc/coreutils.texi (df invocation): Likewise.  Additionally,
explicitly mention the default order of the --output option.