fs: rely on ->iterate_shared to determine f_pos locking

Now that we removed ->iterate we don't need to check for either
->iterate or ->iterate_shared in file_needs_f_pos_lock(). Simply check
for ->iterate_shared instead. This will tell us whether we need to
unconditionally take the lock. Not just does it allow us to avoid
checking f_inode's mode it also actually clearly shows that we're
locking because of readdir.

Signed-off-by: Christian Brauner <brauner@kernel.org>