[PATCH] reduce stack usage in ixgb_ethtool_ioctl

Stack usage reduced from 3024 (!) to 576.
Most of those 3k came from a bug:

#define IXGB_REG_DUMP_LEN  136*sizeof(uint32_t)
                              ^^^^^^^^^^^^^^^^^
...
uint32_t regs_buff[IXGB_REG_DUMP_LEN];

Bug fixed. Two large on-stack variables moved to kmalloc space.

Stack usage is still high because gcc will
allocate too much space for these cases:

        case ETHTOOL_GSET:{
                        struct ethtool_cmd ecmd = { ETHTOOL_GSET };
                        ixgb_ethtool_gset(adapter, &ecmd);
                        if (copy_to_user(addr, &ecmd, sizeof(ecmd)))
                                return -EFAULT;
                        return 0;
                }
        case ETHTOOL_SSET:{
                        struct ethtool_cmd ecmd;
                        if (copy_from_user(&ecmd, addr, sizeof(ecmd)))
                                return -EFAULT;
                        return ixgb_ethtool_sset(adapter, &ecmd);
                }

There will be space for _two_ ecmd's on stack.

Shall it be worked around with ugly union of structs
or we'll just wait for better gcc?

Compile-tested only.
--
vda