xfs: set the unwritten bit in rmap lookup flags in xchk_bmap_get_rmapextents

[ Upstream commit 5dda3897fd90783358c4c6115ef86047d8c8f503 ]

When the bmbt scrubber is looking up rmap extents, we need to set the
extent flags from the bmbt record fully.  This will matter once we fix
the rmap btree comparison functions to check those flags correctly.

Fixes: d852657c ("xfs: cross-reference reverse-mapping btree")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>