Allow newer ldiskfs with d_rehash_cond() and d_move_locked() to build
against older kernels with the older __d_rehash() and __d_move() functions.

Minor reformatting of configure.ac to match GNU autoconf coding style.
http://www.gnu.org/software/autoconf/manual/html_node/Coding-Style.html

b=14714
i=scjody