tracefs: Use d_inode() helper function to get the dentry inode

Instead of referencing the inode from a dentry via dentry->d_inode, use
the helper function d_inode(dentry) instead. This is the considered the
correct way to access it.

Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Reported: https://lore.kernel.org/all/20211208104454.nhxyvmmn6d2qhpwl@wittgenstein/


Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>