xfs: remove SYNC_IOWAIT

We want to wait for all I/O to finish when we do data integrity syncs.  So
there is no reason to keep SYNC_WAIT separate from SYNC_IOWAIT.  This
causes a little change in behaviour for the ENOSPC flushing code which now
does a second submission and wait of buffered I/O, but that should finish
ASAP as we already did an asynchronous writeout earlier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>