Refactor btree allocation to a common helper.
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NChandan Babu R <chandan.babu@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <dchinner@redhat.com>