f2fs: introduce device aliasing file

f2fs: introduce device aliasing file

F2FS should understand how the device aliasing file works and support
deleting the file after use. A device aliasing file can be created by
mkfs.f2fs tool and it can map the whole device with an extent, not
using node blocks. The file space should be pinned and normally used for
read-only usages.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>