f2fs: implement iomap operations

Implement 'struct iomap_ops' for f2fs, in preparation for making f2fs
use iomap for direct I/O.

Note that this may be used for other things besides direct I/O in the
future; however, for now I've only tested it for direct I/O.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>