common/populate: refactor caching of metadumps to a helper

common/populate: refactor caching of metadumps to a helper

Hoist out of _scratch_populate_cached all the code that we use to save a
metadump of the populated filesystem.  We're going to make this more
involved for XFS in the next few patches so that we can take advantage
of the new support for external devices in metadump/mdrestore.

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