xfs: split xfs_inobt_insert_sprec

Split the finobt version that never merges and uses a different cursor
out of xfs_inobt_insert_sprec to prepare for removing xfs_btnum_t.

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>