xfs_db: make listdir more generally useful

xfs_db: make listdir more generally useful

Enhance the current directory entry iteration code in xfs_db to be more
generally useful by allowing callers to pass around a transaction, a
callback function, and a private pointer.  This will be used in the next
patch to iterate directories when we want to copy their contents out of
the filesystem into a directory.

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