nvme-tcp: set the STABLE_WRITES flag when data digests are enabled

nvme-tcp: set the STABLE_WRITES flag when data digests are enabled

There was a few false alarms sighted on target side about wrong data
digest while performing high throughput load to XFS filesystem shared
through NVMoF TCP.

This flag tells the rest of the kernel to ensure that the data buffer
does not change while the write is in flight.  It incurs a performance
penalty, so only enable it when it is actually needed, i.e. when we are
calculating data digests.

Although even with this change in place, ext2 users can steel experience
false positives, as ext2 is not respecting this flag. This may be apply
to vfat as well.

Signed-off-by: Mikhail Skorzhinskii <mskorzhinskiy@solarflare.com>
Signed-off-by: Mike Playle <mplayle@solarflare.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>