ext4: factor out htree rep invariant check

This patch moves some debugging code which is used to validate the
hash tree node when doing a binary search of an htree node into a
separate function, which is disabled by default (since it is only used
by developers when they are modifying the htree code paths).

In addition to cleaning up the code to make it more maintainable, it
silences a Clang compiler warning when -Wunreachable-code-aggressive
is enabled.  (There is no plan to enable this warning by default, since
there it has far too many false positives; nevertheless, this commit
reduces one of the many false positives by one.)

Signed-off-by: Vinicius Tinti <viniciustinti@gmail.com>
Link: https://lore.kernel.org/r/20210202162837.129631-1-viniciustinti@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>