dm writecache: don't split bios when overwriting contiguous cache content

dm writecache: don't split bios when overwriting contiguous cache content

[ Upstream commit ee50cc19d80e9b9a8283d1fb517a778faf2f6899 ]

If dm-writecache overwrites existing cached data, it splits the
incoming bio into many block-sized bios. The I/O scheduler does merge
these bios into one large request but this needless splitting and
merging causes performance degradation.

Fix this by avoiding bio splitting if the cache target area that is
being overwritten is contiguous.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>