xfs: avoid redundant checks when options is empty

When options passed to xfs_parseargs() is NULL the checks performed
after taking the branch are made with the initial values of dsunit,
dswidth and iosizelog. But all the checks do nothing in this case
so return immediately instead.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>