xfs_mdrestore: refactor open-coded fd/is_file into a structure

xfs_mdrestore: refactor open-coded fd/is_file into a structure

Create an explicit object to track the fd and flags associated with a
device onto which we are restoring metadata, and use it to reduce the
amount of open-coded arguments to ->restore.  This avoids some grossness
in the next patch.

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