ext4: support GETFSMAP ioctls

Support the GETFSMAP ioctls so that we can use the xfs free space
management tools to probe ext4 as well. Note that this is a partial
implementation -- we only report

ext4: support GETFSMAP ioctls

Support the GETFSMAP ioctls so that we can use the xfs free space
management tools to probe ext4 as well. Note that this is a partial
implementation -- we only report fixed-location metadata and free space;
everything else is reported as "unknown".

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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