xfs: don't use a different allocsice for -o wsync

The -o wsync allocsize overwrite overwrite was part of a special hack
for NFSv2 servers in IRIX and has no real purpose in modern Linux, so
remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>