xfs: remove the EXPERIMENTAL warning for pNFS

The pNFS layout support has been around for 10 years without major
issues, drop the EXPERIMENTAL warning.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>