9p p9mode2perm: remove useless strlcpy and check sscanf return code

This is also a checkpatch warning fix but this one might have implications
so keeping it separate

Link: http://lkml.kernel.org/r/20211102134608.1588018-5-dominique.martinet@atmark-techno.com
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>