xfs_db: add command to copy directory trees out of filesystems

xfs_db: add command to copy directory trees out of filesystems

Aheada of deprecating V4 support in the kernel, let's give people a way
to extract their files from a filesystem without needing to mount.  The
libxfs code won't be removed from the kernel until 2030 and xfsprogs
effectively builds with XFS_SUPPORT_V4=y so that'll give us five years
of releases for archaeologists to draw from.  Also, doing this in
userspace gives people a way to recover files in an unprivileged
container for better safety.

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