xfs: fix flags argument to rmap lookup when converting shared file rmaps

Pass the same oldext argument (which contains the existing rmapping's
unwritten state) to xfs_rmap_lookup_le_range at the start of
xfs_rmap_convert_shared.  At this point in the code, flags is zero,
which means that we perform lookups using the wrong key.

Fixes: 3f165b33 ("xfs: convert unwritten status of reverse mappings for shared files")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>