x86: limit the fs segment to the pointer size

The fs segment is only used to get the global data pointer.
If it is accessed beyond sizeof(new_gd->arch.gd_addr), it is a bug.

To specify the byte-granule limit size, drop the G bit, so the
flag field is 0x8093 instead of 0xc093, and set the limit field
to sizeof(new_gd->arch.gd_addr) - 1.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fixed the comments about FS segement]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>