f2fs: clean up f2fs_truncate_partial_cluster()

Clean up codes as below:
- avoid unnecessary "err > 0" check condition
- use "1 << log_cluster_size" instead of F2FS_I(inode)->i_cluster_size

No logic changes.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>