jbd2: rename functions which initialize slab caches

This patch renames functions initializing the slab caches for the
journal head and handle structures to so they are consistent with the
names of the corresponding functions which destroys those slab caches.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>