generic/735: skip this test if we cannot finsert at pos 1M

Add a _require_congruent_file_oplen to screen out filesystem
configurations that can't start a finsert operation at file pos 1M
because the fs block size isn't congruent with 1048576.  For example,
xfs realtime with 28k rt extents.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>