vfs: stop using user_path_at_empty in do_readlinkat

vfs: stop using user_path_at_empty in do_readlinkat

It is the only consumer and it saddles getname_flags with an argument set
to NULL by everyone else.

Instead the routine can do the empty check on its own.

Then user_path_at_empty can get retired and getname_flags can lose the
argument.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240604155257.109500-2-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>