Fix regression in POSIX mode behavior

Commit 235a85657 introduced a regression in evaluation of POSIX modes
that require group DENY entries in the internal ZFS ACL. An example
of such a POSX mode is 007. When write_implies_delete_child is set,
then ACE_WRITE_DATA is added to `wanted_dirperms` in prior to calling
zfs_zaccess_common(). This occurs is zfs_zaccess_delete().

Unfortunately, when zfs_zaccess_aces_check hits this particular DENY
ACE, zfs_groupmember() is checked to determine whether access should be
denied, and since zfs_groupmember() always returns B_TRUE on Linux and
so this check is failed, resulting ultimately in EPERM being returned.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Andrew Walker <awalker@ixsystems.com>
Closes #11760

module/zfs/zfs_fuid.c

@@ -728,7 +728,6 @@ zfs_fuid_info_free(zfs_fuid_info_t *fuidp)
 boolean_t
 zfs_groupmember(zfsvfs_t *zfsvfs, uint64_t id, cred_t *cr)
 {
-#ifdef HAVE_KSID
 	uid_t		gid;
 
 #ifdef illumos
@@ -773,9 +772,6 @@ zfs_groupmember(zfsvfs_t *zfsvfs, uint64_t id, cred_t *cr)
 	 */
 	gid = zfs_fuid_map_id(zfsvfs, id, cr, ZFS_GROUP);
 	return (groupmember(gid, cr));
-#else
-	return (B_TRUE);
-#endif
 }
 
 void