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/jfs/ioctl.c b/fs/jfs/ioctl.c
index 6f98a18..73d9eaa 100644
--- a/fs/jfs/ioctl.c
+++ b/fs/jfs/ioctl.c
@@ -68,7 +68,7 @@
 		unsigned int oldflags;
 		int err;
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;