btrfs: optimize split page write in btrfs_set_token_##bits

The fallback path calls helper write_extent_buffer to do write of the
data spanning two extent buffer pages. As the size is known, we can do
the write directly in two steps.  This removes one function call and
compiler can optimize memcpy as the sizes are known at compile time. The
cached token address is set to the second page.

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