signals: jffs2: fix the wrong usage of disallow_signal()

signals: jffs2: fix the wrong usage of disallow_signal()

jffs2_garbage_collect_thread() does disallow_signal(SIGHUP) around
jffs2_garbage_collect_pass() and the comment says "We don't want SIGHUP to
interrupt us".

But disallow_signal() can't ensure that jffs2_garbage_collect_pass() won't
be interrupted by SIGHUP, the problem is that SIGHUP can be already
pending when disallow_signal() is called, and in this case any
interruptible sleep won't block.

Note: this is in fact because disallow_signal() is buggy and should be
fixed, see the next changes.

But there is another reason why disallow_signal() is wrong: SIG_IGN set by
disallow_signal() silently discards any SIGHUP which can be sent before
the next allow_signal(SIGHUP).

Change this code to use sigprocmask(SIG_UNBLOCK/SIG_BLOCK, SIGHUP).  This
even matches the old (and wrong) semantics allow/disallow had when this
logic was written.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>