xfs: refactor the geometry structure filling function

Refactor the geometry structure filling function to use the superblock
to fill the fields.  While we're at it, make the function less indenty
and use some whitespace to make the function easier to read.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>