xfs: pass a 'bool is_finobt' to xfs_inobt_insert

This is one of the last users of xfs_btnum_t and can only designate
either the inobt or finobt.  Replace it with a simple bool.

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>