lib/Kconfig.debug: make TEST_KMOD depend on PAGE_SIZE_LESS_THAN_256KB

Commit b05fbcc3 ("btrfs: disable build on platforms having page size
256K") disabled btrfs for configurations that used a 256kB page size.
However, it did not fully solve the problem because CONFIG_TEST_KMOD
selects CONFIG_BTRFS, which does not account for the dependency.  This
results in a Kconfig warning and the failed BUILD_BUG_ON error
returning.

  WARNING: unmet direct dependencies detected for BTRFS_FS
    Depends on [n]: BLOCK [=y] && !PPC_256K_PAGES && !PAGE_SIZE_256KB [=y]
    Selected by [m]:
    - TEST_KMOD [=m] && RUNTIME_TESTING_MENU [=y] && m && MODULES [=y] && NETDEVICES [=y] && NET_CORE [=y] && INET [=y] && BLOCK [=y]

To resolve this, add CONFIG_PAGE_SIZE_LESS_THAN_256KB as a dependency of
CONFIG_TEST_KMOD so there is no more invalid configuration or build
errors.

Link: https://lkml.kernel.org/r/20211129230141.228085-4-nathan@kernel.org
Fixes: b05fbcc3

 ("btrfs: disable build on platforms having page size 256K")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Chris Mason <clm@fb.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>