freezer: PF_FREEZER_NOSIG should be cleared along with PF_NOFREEZE

freezer: PF_FREEZER_NOSIG should be cleared along with PF_NOFREEZE

This patch is only for pre-v3.3 stable trees which backported
b40a7959 "freezer: exec should clear PF_NOFREEZE along with PF_KTHREAD".
v3.3+ doesn't need this fix.

b40a7959 is the trivial bugfix, but unfortunately I forgot that
until 34b087e4 "freezer: kill unused set_freezable_with_signal()"
there were another only-for-kernel-threads flag, PF_FREEZER_NOSIG,
which should be cleared as well.

See https://bugs.launchpad.net/ubuntu/+source/v86d/+bug/1080530
The freezer fails because it expects that a PF_FREEZER_NOSIG task
doesn't need a signal. Before b40a7959 it wrongly succeeds leaving
the PF_NOFREEZE | PF_FREEZER_NOSIG task unfrozen.

Reported-and-tested-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[bwh: Don't touch PF_FORKNOEXEC; it's cleared elsewhere]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>