NFSD: Make FILE_SYNC WRITEs comply with spec

NFSD: Make FILE_SYNC WRITEs comply with spec

commit e3e8e176ca4876e6212582022ad80835dddc9de4 upstream.

Mike noted that when NFSD responds to an NFS_FILE_SYNC WRITE, it
does not also persist file time stamps. To wit, Section 18.32.3
of RFC 8881 mandates:

> The client specifies with the stable parameter the method of how
> the data is to be processed by the server. If stable is
> FILE_SYNC4, the server MUST commit the data written plus all file
> system metadata to stable storage before returning results. This
> corresponds to the NFSv2 protocol semantics. Any other behavior
> constitutes a protocol violation. If stable is DATA_SYNC4, then
> the server MUST commit all of the data to stable storage and
> enough of the metadata to retrieve the data before returning.

Commit 3f3503adb332 ("NFSD: Use vfs_iocb_iter_write()") replaced:

-		flags |= RWF_SYNC;

with:

+		kiocb.ki_flags |= IOCB_DSYNC;

which appears to be correct given:

	if (flags & RWF_SYNC)
		kiocb_flags |= IOCB_DSYNC;

in kiocb_set_rw_flags(). However the author of that commit did not
appreciate that the previous line in kiocb_set_rw_flags() results
in IOCB_SYNC also being set:

	kiocb_flags |= (__force int) (flags & RWF_SUPPORTED);

RWF_SUPPORTED contains RWF_SYNC, and RWF_SYNC is the same bit as
IOCB_SYNC. Reviewers at the time did not catch the omission.

Reported-by: Mike Snitzer <snitzer@kernel.org>
Closes: https://lore.kernel.org/linux-nfs/20251018005431.3403-1-cel@kernel.org/T/#t
Fixes: 3f3503adb332 ("NFSD: Use vfs_iocb_iter_write()")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>