db: add finobt support to metadump

db: add finobt support to metadump

Include the free inode btree in metadump images. If the source fs
is finobt-enabled, run an additional scan_btree() of the finobt.
Since the private 'agi' scanfunc_ino() parameter is unused, change
the private parameter to a flag that indicates whether the current
scan is for the inobt or finobt. If the latter, we skip copying the
actual inode chunks as this work is already performed by the inobt
scan.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>