libfrog: support editing filesystem property sets

Add some library functions so that spaceman and scrub can share the same
code to edit and retrieve filesystem properties.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dave Chinner <dchinner@redhat.com>
5 files changed