xfs: move xfs_parseargs() validation to a helper

Move the validation code of xfs_parseargs() into a helper for later
use within the mount context methods.

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