btrfs: add special case to setget helpers for 64k pages

On 64K pages the size of the extent_buffer::pages array is 1 and
compilation with -Warray-bounds warns due to

  kaddr = page_address(eb->pages[idx + 1]);

when reading byte range crossing page boundary.

This does never actually overflow the array because on 64K because all
the data fit in one page and bounds are checked by check_setget_bounds.

To fix the reported overflows and warnings add a compile-time condition
that will allow compiler to eliminate the dead code that reads from the
idx + 1 page.

Link: https://lore.kernel.org/lkml/20210623083901.1d49d19d@canb.auug.org.au/


CC: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>