docs: path-lookup: update path_mountpoint() part

path_mountpoint() doesn't exist anymore. Have been folded
into path_lookup_at when flag is set with LOOKUP_MOUNTPOINT.
Check commit: commit 161aff1d

 ("LOOKUP_MOUNTPOINT: fold
path_mountpointat() into path_lookupat()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-4-foxhlchen@gmail.com


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