ext4: Rename block group descriptor table from gd to bgd

On x86 machines gd is unfortunately a #define, so we should avoid using
gd for anything. This patch changes uses of gd to bgd so that ext4fs
can be used on x86.

A better fix would be to remove the #define in x86, but I'm not sure
how to do that.

Signed-off-by: Simon Glass <sjg@chromium.org>