xfs: refactor xfs_ialloc_btree.c to support multiple inobt numbers

The introduction of the free inode btree (finobt) requires that
xfs_ialloc_btree.c handle multiple trees. Refactor xfs_ialloc_btree.c
so the caller specifies the btree type on cursor initialization to
prepare for addition of the finobt.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>