f2fs: fix to detect temporary name of multimedia file

Some applications may create multimeida file with temporary name like
'*.jpg.tmp' or '*.mp4.tmp', then rename to '*.jpg' or '*.mp4'.

Now, f2fs can only detect multimedia filename with specified format:
"filename + '.' + extension", so it will make f2fs missing to detect
multimedia file with special temporary name, result in failing to set
cold flag on file.

This patch enhances detection flow for enabling lookup extension in the
middle of temporary filename.

Reported-by: Xue Liu <liuxueliu.liu@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>