direct-io: Handle O_(D)SYNC AIO

direct-io: Handle O_(D)SYNC AIO

Call generic_write_sync() from the deferred I/O completion handler if
O_DSYNC is set for a write request.  Also make sure various callers
don't call generic_write_sync if the direct I/O code returns
-EIOCBQUEUED.

Based on an earlier patch from Jan Kara <[email protected]> with updates from
Jeff Moyer <[email protected]> and Darrick J. Wong <[email protected]>.

Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Al Viro <[email protected]>