Freezer: Fix JFFS2 garbage collector freezing issue (rev. 2)

Fix breakage caused by commit d5d8c597


"freezer: do not send signals to kernel threads" in
jffs2_garbage_collect_thread() that assumed it would be sent signals
by the freezer.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Pete MacKay <armlinux@architechnical.net>
Signed-off-by: Len Brown <len.brown@intel.com>