pNFS/filelayout: Fix coalescing test for single DS

stable inclusion
from stable-v5.10.165
commit c7c36bb6eafd3f349d2371a1c6f2a3a72400acbc
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7T7G4

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7c36bb6eafd3f349d2371a1c6f2a3a72400acbc



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[ Upstream commit a6b9d2fa ]

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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>