direct-io: allow direct writes to empty inodes

[ Upstream commit 8b9433eb4de3c26a9226c981c283f9f4896ae030 ]

On a DIO_SKIP_HOLES filesystem, the ->get_block() method is currently
not allowed to create blocks for an empty inode.  This confusion comes
from trying to bit shift a negative number, so check the size of the
inode first.

The problem is most visible for hfsplus, because the fallback to
buffered I/O doesn't happen and the write fails with EIO.  This is in
part the fault of the module, because it gives a wrong return value on
->get_block(); that will be fixed in a separate patch.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>