f2fs: fix dropping inmemory pages in a wrong time

When one reader closes its file while the other writer is doing atomic writes,
f2fs_release_file drops atomic data resulting in an empty commit.
This patch fixes this wrong commit problem by checking openess of the file.

 Process0                       Process1
 				open file
 start atomic write
 write data
 read data
				close file
				f2fs_release_file()
				clear atomic data
 commit atomic write

Reported-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>