xfs_scrub: hoist scrub retry loop to scrub_item_check_file

For metadata check calls, use the ioctl retry and freeze permission
tracking in scrub_item that we created in the last patch.  This enables
us to move the check retry loop out of xfs_scrub_metadata and into its
caller to remove a long backwards jump, and gets us closer to
vectorizing scrub calls.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>