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: 86b54881 ("debugfs: Add write support to debugfs_create_str()")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309091933.BRWlSnCq-lkp@intel.com/Signed-off-by: Mike Tipton <quic_mdtipton@quicinc.com>
Link: https://lore.kernel.org/r/20230922134512.5126-1-quic_mdtipton@quicinc.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/file.c

@@ -939,7 +939,7 @@ static ssize_t debugfs_write_file_str(struct file *file, const char __user *user
 	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);