9p: fix misuse of sscanf() in v9fs_stat2inode()

1) sscanf() return value needs to be checked, damnit
2) sscanf() is perfectly capable of checking for fixed prefix,
no need for that %13s + strncmp with constant string.
3) st->extension is a valid string; no need for voodoo with
str*cpy() there.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>