direct-io: Prevent NULL pointer access in submit_page_section

direct-io: Prevent NULL pointer access in submit_page_section

In the code added to function submit_page_section by commit b1058b981,
sdio->bio can currently be NULL when calling dio_bio_submit.  This then
leads to a NULL pointer access in dio_bio_submit, so check for a NULL
bio in submit_page_section before trying to submit it instead.

Fixes xfstest generic/250 on gfs2.

Cc: [email protected] # v3.10+
Signed-off-by: Andreas Gruenbacher <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Al Viro <[email protected]>