xfsprog: add mkfs.xfs sb v4 support for dirent filetype field

xfsprog: add mkfs.xfs sb v4 support for dirent filetype field

Add directory inode type feature to mkfs.xfs and its manual page.

In sb v4, "mkfs.xfs -n ftype=1" turns on the feature.

The feature is automatically turned on for "-m crc=1", but reject
the use of the "-n ftype=0|1" with the "-m crc=1" option.

Signed-off-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>