net/mlx5e: Ensure macsec_rule is always initiailized in macsec_fs_{r,t}x_add_rule()

net/mlx5e: Ensure macsec_rule is always initiailized in macsec_fs_{r,t}x_add_rule()

Clang warns:

  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:539:6: error: variable 'macsec_rule' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
          if (err)
              ^~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:598:9: note: uninitialized use occurs here
          return macsec_rule;
                ^~~~~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:539:2: note: remove the 'if' if its condition is always false
          if (err)
          ^~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:523:38: note: initialize the variable 'macsec_rule' to silence this warning
          union mlx5e_macsec_rule *macsec_rule;
                                              ^
                                              = NULL
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:1131:6: error: variable 'macsec_rule' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
          if (err)
              ^~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:1215:9: note: uninitialized use occurs here
          return macsec_rule;
                ^~~~~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:1131:2: note: remove the 'if' if its condition is always false
          if (err)
          ^~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_accel/macsec_fs.c:1118:38: note: initialize the variable 'macsec_rule' to silence this warning
          union mlx5e_macsec_rule *macsec_rule;
                                              ^
                                              = NULL
  2 errors generated.

If macsec_fs_{r,t}x_ft_get() fail, macsec_rule will be uninitialized.
Initialize it to NULL at the top of each function so that it cannot be
used uninitialized.

Fixes: e467b283ffd5 ("net/mlx5e: Add MACsec TX steering rules")
Fixes: 3b20949cb21b ("net/mlx5e: Add MACsec RX steering rules")
Link: https://github.com/ClangBuiltLinux/linux/issues/1706
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Link: https://lore.kernel.org/r/20220911085748.461033-1-nathan@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>