Currently we don't overwrite the flags field in the iomap in
xfs_bmbt_to_iomap. This works fine with 0-initialized iomaps on stack,
but is harmful once we want to be able to reuse an iomap in the
writeback code. Replace the shared parameter with a set of initial
flags an thus ensures the flags field is always reinitialized.
Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>