xfs: Accept filesystem with sparse inodes

xfs: Accept filesystem with sparse inodes

The sparse inode metadata format became a mkfs.xfs default in
xfsprogs-4.16.0, and such filesystems are now rejected by grub as
containing an incompatible feature.

In essence, this feature allows xfs to allocate inodes into fragmented
freespace.  (Without this feature, if xfs could not allocate contiguous
space for 64 new inodes, inode creation would fail.)

In practice, the disk format change is restricted to the inode btree,
which as far as I can tell is not used by grub.  If all you're doing
today is parsing a directory, reading an inode number, and converting
that inode number to a disk location, then ignoring this feature
should be fine, so I've added it to XFS_SB_FEAT_INCOMPAT_SUPPORTED

I did some brief testing of this patch by hacking up the regression
tests to completely fragment freespace on the test xfs filesystem, and
then write a large-ish number of inodes to consume any existing
contiguous 64-inode chunk.  This way any files the grub tests add and
traverse would be in such a fragmented inode allocation.  Tests passed,
but I'm not sure how to cleanly integrate that into the test harness.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Chris Murphy <lists@colorremedies.com>