reiser4: fix reiser4_post_{commit,write_back}_hook() and their invocations.

- let all hooks be called from one place (reiser4_write_logs())
- don't call reiser4_post_write_back_hook() twice
- fix reiser4_post_write_back_hook(): call the correct method of space allocator

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>