f2fs: clear atomic_write_task in f2fs_abort_atomic_write()

Otherwise, last .atomic_write_task will be remained in structure
f2fs_inode_info, resulting in aborting atomic_write accidentally
in race case. Meanwhile, clear original_i_size as well.

Fixes: 7a10f017 ("f2fs: don't give partially written atomic data from process crash")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>