f2fs: clean up f2fs_abort_atomic_write()

f2fs_abort_atomic_write() has checked whether current inode is
atomic_write one or not, it's redundant to check in its caller,
remove it for cleanup.

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