docs: path-lookup: update get_link() ->follow_link description

get_link() is merged into pick_link(). i_op->follow_link is
replaced with i_op->get_link(). get_link() can return ERR_PTR(0)
which equals NULL.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-12-foxhlchen@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>