xfs: create predicate to determine if cursor is at inode root level

Create a predicate to decide if the given cursor and level point to the
root block in the inode immediate area instead of a disk block, and get
rid of the open-coded logic everywhere.

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