gfs2: Allow append and immutable bits to coexist

Before this patch, function do_gfs2_set_flags checked if the append
and immutable flags were being set while already set. If so, error -EPERM
was given. There's no reason why these two flags should be mutually
exclusive, and if you set them separately, you will, in essence, set
one while it is already set. For example:

chattr +a /mnt/gfs2/file1
chattr +i /mnt/gfs2/file1

The first command sets the append-only flag. Since they are additive,
the second command sets the immutable flag AND append-only flag,
since they both coexist in i_diskflags. So the second command should
not return an error. This bug caused xfstests generic/545 to fail.

This patch simply removes the invalid checks.
I also eliminated an unused parm from do_gfs2_set_flags.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>