gfs2: Silence "suspicious RCU usage in gfs2_permission" warning

Commit 0abd1557 added rcu_dereference() for dereferencing ip->i_gl
in gfs2_permission.  This now causes lockdep to complain when
gfs2_permission is called in non-RCU context:

    WARNING: suspicious RCU usage in gfs2_permission

Switch to rcu_dereference_check() and check for the MAY_NOT_BLOCK flag
to shut up lockdep when we know that dereferencing ip->i_gl is safe.

Fixes: 0abd1557 ("gfs2: fix an oops in gfs2_permission")
Reported-by:  <syzbot+3e5130844b0c0e2b4948@syzkaller.appspotmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>