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/gfs2/file.c b/fs/gfs2/file.c
index ce36a56..28fc6e3 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -223,7 +223,7 @@
 	int error;
 	u32 new_flags, flags;
 
-	error = mnt_want_write(filp->f_path.mnt);
+	error = mnt_want_write_file(filp);
 	if (error)
 		return error;