xfs: fix potential memory leak in mount option parsing

When specifying string type mount option (e.g., logdev)
several times in a mount, current option parsing may
cause memory leak. Hence, call kfree for previous one
in this case.

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>