btrfs: disable build on platforms having page size 256K

With a config having PAGE_SIZE set to 256K, BTRFS build fails
with the following message

  include/linux/compiler_types.h:326:38: error: call to
  '__compiletime_assert_791' declared with attribute error:
  BUILD_BUG_ON failed: (BTRFS_MAX_COMPRESSED % PAGE_SIZE) != 0

BTRFS_MAX_COMPRESSED being 128K, BTRFS cannot support platforms with
256K pages at the time being.

There are two platforms that can select 256K pages:
 - hexagon
 - powerpc

Disable BTRFS when 256K page size is selected. Supporting this would
require changes to the subpage mode that's currently being developed.
Given that 256K is many times larger than page sizes commonly used and
for what the algorithms and structures have been tuned, it's out of
scope and disabling build is a reasonable option.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>