nfsd4: remove ACE4_IDENTIFIER_GROUP flag from GROUP@ entry

RFC 3530 says "ACE4_IDENTIFIER_GROUP flag MUST be ignored on entries
with these special identifiers.  When encoding entries with these
special identifiers, the ACE4_IDENTIFIER_GROUP flag SHOULD be set to
zero."  It really shouldn't matter either way, but the point is that
this flag is used to distinguish named users from named groups (since
unix allows a group to have the same name as a user), so it doesn't
really make sense to use it on a special identifier such as this.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>