debugfs: Fix __rcu type comparison warning

Sparse reports the following:

fs/debugfs/file.c:942:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
fs/debugfs/file.c:942:9: sparse: char [noderef] __rcu *
fs/debugfs/file.c:942:9: sparse: char *

rcu_assign_pointer() expects that it's assigning to pointers annotated
with __rcu. We can't annotate the generic struct file::private_data, so
cast it instead.

Fixes: 86b5488121db ("debugfs: Add write support to debugfs_create_str()")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Mike Tipton <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

fs/debugfs/file.c

···
 	new[pos + count] = '\0';
 	strim(new);
 
-	rcu_assign_pointer(*(char **)file->private_data, new);
+	rcu_assign_pointer(*(char __rcu **)file->private_data, new);
 	synchronize_rcu();
 	kfree(old);