In readdirplus, don't keep such pointers but store the file attributes
in a variable instead until they are needed. Change nfsm_loadattr*
a bit so it can accept a direct pointer to an nfs_fattr structure.
struct member cn_nameptr 'const', since they should never be used to
modify the path name. (Only the pathname buffer, cn_pnbuf, should be
modified.) Propagate the const poisoning to code that uses the namei
and componentname structs.