xfs: fix fsmap for internal zoned devices

Filesystems with an internal zoned rt section use xfs_rtblock_t values
that are relative to the start of the data device.  When fsmap reports
on internal rt sections, it reports the space used by the data section
as "OWN_FS".

Unfortunately, the logic for resuming a query isn't quite right, so
xfs/273 fails because it stress-tests GETFSMAP with a single-record
buffer.  If we enter the "report fake space as OWN_FS" block with a
nonzero key[0].fmr_length, we should add that to key[0].fmr_physical
and recheck if we still need to emit the fake record.  We should /not/
just return 0 from the whole function because that prevents all rmap
record iteration.

If we don't enter that block, the resumption is still wrong.
keys[*].fmr_physical is a reflection of what we copied out to userspace
on a previous query, which means that it already accounts for rgstart.
It is not correct to add rtstart_daddr when computing start_rtb or
end_rtb, so stop that.

While we're at it, add a xfs_has_zoned to make it clear that this is a
zoned filesystem thing.

Fixes: e50ec7fa ("xfs: enable fsmap reporting for internal RT devices")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>