f2fs: fix to release node block count in error path of f2fs_new_node_page()

It missed to call dec_valid_node_count() to release node block count
in error path, fix it.

Fixes: 141170b7 ("f2fs: fix to avoid use f2fs_bug_on() in f2fs_new_node_page()")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>