btrfs: optimize split page read in btrfs_get_##bits

The helper read_extent_buffer is called to do read of the data spanning
two extent buffer pages. As the size is known, we can do the read
directly in two steps.  This removes one function call and compiler can
optimize memcpy as the sizes are known at compile time.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>