xfs_repair: define an in-memory btree for storing refcount bag info

xfs_repair: define an in-memory btree for storing refcount bag info

Create a new in-memory btree type so that we can store refcount bag info
in a much more memory-efficient format.  Note that the xfs_repair rcbag
btree stores inode numbers (unlike the kernel rcbag btree) because
xfs_repair needs to compute the bitmap of inodes that must have the
reflink iflag set.

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