f2fs: move ioctl interface definitions to separated file

Like other filesystem does, we introduce a new file f2fs.h in path of
include/uapi/linux/, and move f2fs-specified ioctl interface definitions
to that file, after then, in order to use those definitions, userspace
developer only need to include the new header file rather than
copy & paste definitions from fs/f2fs/f2fs.h.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>