cachefiles: Fix oops in vfs_mkdir from cachefiles_get_directory

Commit c54b386 ("VFS: Change vfs_mkdir() to return the dentry.")
changed cachefiles_get_directory, replacing "subdir" with a ERR_PTR
from the result of cachefiles_inject_write_error, which is either 0
or some error code.  This causes an oops when the resulting pointer
is passed to vfs_mkdir.

Use a similar pattern to what is used earlier in the function; replace
subdir with either the return value from vfs_mkdir, or the ERR_PTR
of the cachefiles_inject_write_error() return value, but only if it
is non zero.

Fixes: c54b386 ("VFS: Change vfs_mkdir() to return the dentry.")
cc: netfs@lists.linux.dev
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Link: https://lore.kernel.org/r/20250325125905.395372-1-marc.dionne@auristor.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Marc Dionne

fs/cachefiles/namei.c

@@ -128,10 +128,11 @@ struct dentry *cachefiles_get_directory(struct cachefiles_cache *cache,
 		ret = security_path_mkdir(&path, subdir, 0700);
 		if (ret < 0)
 			goto mkdir_error;
-		subdir = ERR_PTR(cachefiles_inject_write_error());
-		if (!IS_ERR(subdir))
+		ret = cachefiles_inject_write_error();
+		if (ret == 0)
 			subdir = vfs_mkdir(&nop_mnt_idmap, d_inode(dir), subdir, 0700);
-		ret = PTR_ERR(subdir);
+		else
+			subdir = ERR_PTR(ret);
 		if (IS_ERR(subdir)) {
 			trace_cachefiles_vfs_error(NULL, d_inode(dir), ret,
 						   cachefiles_trace_mkdir_error);