xfs: add a incompat feature bit for parent pointers

xfs: add a incompat feature bit for parent pointers

Source kernel commit: 5f98ec1cb5c264e4815e21d632ee0b3d6e700e3d

Create an incompat feature bit and a fs geometry flag so that we can
enable the feature in the ondisk superblock and advertise its existence
to userspace.

Signed-off-by: Mark Tinguely <mark.tinguely@oracle.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>