xfs_admin: clean up string quoting

xfs_admin: clean up string quoting

Clean up the string quoting in this script so that we don't trip over
users feeding us arguments like "/dev/sd ha ha ha lol".

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>