pNFS/filelayout: Fix coalescing test for single DS

[ Upstream commit a6b9d2fa0024e7e399c26facd0fb466b7396e2b9 ]

When there is a single DS no striping constraints need to be placed on
the IO. When such constraint is applied then buffered reads don't
coalesce to the DS's rsize.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>