RDMA/mlx5: fix build error with INFINIBAND_USER_ACCESS=n

RDMA/mlx5: fix build error with INFINIBAND_USER_ACCESS=n

The mlx5_ib_fs_add_op_fc/mlx5_ib_fs_remove_op_fc functions are only
available when user access is enabled, without that we run into a link
error:

ERROR: modpost: "mlx5_ib_fs_add_op_fc" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
ERROR: modpost: "mlx5_ib_fs_remove_op_fc" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!

Conditionally compiling the newly added code section makes it build,
though this is probably not a correct fix.

Fixes: 0c2415026758 ("RDMA/mlx5: Add modify_op_stat() support")
Link: https://lore.kernel.org/r/20211019061602.3062196-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>