nfsd: assume writeable exportabled filesystems have f_sync

I don't really see how you could claim to support nfsd and not support
fsync somehow.

And in practice a quick look through the exportable filesystems suggests
the only ones without an ->fsync are read-only (efs, isofs, squashfs) or
in-memory (shmem).

Also, performing a write and then returning an error if the sync fails
(as we would do here in the wgather case) seems unhelpful to clients.

Also remove an incorrect comment.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -1020,21 +1020,8 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 	inode = dentry->d_inode;
 	exp   = fhp->fh_export;
 
-	/*
-	 * Request sync writes if
-	 *  -	the sync export option has been set, or
-	 *  -	the client requested O_SYNC behavior (NFSv3 feature).
-	 *  -   The file system doesn't support fsync().
-	 * When NFSv2 gathered writes have been configured for this volume,
-	 * flushing the data to disk is handled separately below.
-	 */
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
 
-	if (!file->f_op->fsync) {/* COMMIT3 cannot work */
-	       stable = 2;
-	       *stablep = 2; /* FILE_SYNC */
-	}
-
 	if (!EX_ISSYNC(exp))
 		stable = 0;
 	if (stable && !use_wgather) {