switch a bunch of places to mnt_want_write_file()

it's both faster (in case when file has been opened for write) and cleaner.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ocfs2/ioctl.c b/fs/ocfs2/ioctl.c
index 726ff26..892ace2 100644
--- a/fs/ocfs2/ioctl.c
+++ b/fs/ocfs2/ioctl.c
@@ -906,7 +906,7 @@
 		if (get_user(flags, (int __user *) arg))
 			return -EFAULT;
 
-		status = mnt_want_write(filp->f_path.mnt);
+		status = mnt_want_write_file(filp);
 		if (status)
 			return status;
 		status = ocfs2_set_inode_attr(inode, flags,