xfs: remove XFS_IFINLINE

Just check for an inline format fork instead of the using the equivalent
in-memory XFS_IFINLINE flag.

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>