xfs: fold xfs_inobt_init_common into xfs_inobt_init_cursor

Make the levels initialization in xfs_inobt_init_cursor conditional
and merge the two helpers.

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