btrfs: add ioctl to export size of global metadata reservation
btrfs filesystem df output will show the size of the metadata space and how much of it is used, and the user assumes that the difference is all usable space. Since that's not actually the case due to the global metadata reservation, we should provide the full picture to the user. This patch adds an ioctl that exports the size of the global metadata reservation so that btrfs filesystem df can report it. Signed-off-by: NJeff Mahoney <jeffm@suse.com> Signed-off-by: NJosef Bacik <jbacik@fusionio.com> Signed-off-by: NChris Mason <clm@fb.com>
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