direct-io: clean up error paths of do_blockdev_direct_IO

In preparation to resort DIO checks, reduce code duplication of error
handling in do_blockdev_direct_IO.

Link: https://lore.kernel.org/r/20201008062620.2928326-2-krisman@collabora.com
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>