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: NJeff Moyer <jmoyer@redhat.com>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NErnesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>