f2fs: avoid unneeded error handling for revoke_entry_slab allocation

In __f2fs_commit_atomic_write(), we will guarantee success of
revoke_entry_slab allocation, so let's avoid unneeded error handling.

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