nfsd: Sanity check the ACL attribute

When an ACL is presented to the NFSv4 server in
Setattr or Verify, parsing of the ACL assumed a
sane acecnt and sane sizes for the "who" strings.
This patch adds sanity checks for these.

The patch also fixes handling of an error
return from nfsrv_dissectacl() for one broken
case.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260111
MFC after: 2 weeks