direct-io: allow direct writes to empty inodes

direct-io: allow direct writes to empty inodes

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 <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Ernesto A. Fernández <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>