xfs: fix the contact address for the sysfs ABI documentation

oss.sgi.com is long dead, refer to the current linux-xfs list instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>