nfs: fix broken handling of the softreval mount option

Turns out that ever since this mount option was added, passing
`softreval` in NFS mount options cancelled all other flags while not
affecting the underlying flag `NFS_MOUNT_SOFTREVAL`.

Fixes: c74dfe97c1 ("NFS: Add mount option 'softreval'")
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/fs_context.c

@@ -517,7 +517,7 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 		if (result.negated)
 			ctx->flags &= ~NFS_MOUNT_SOFTREVAL;
 		else
-			ctx->flags &= NFS_MOUNT_SOFTREVAL;
+			ctx->flags |= NFS_MOUNT_SOFTREVAL;
 		break;
 	case Opt_posix:
 		if (result.negated)