MSI-HOWTO.txt: remove reference on IRQF_DISABLED

The IRQF_DISABLED is a NOOP and scheduled to be removed.  According to Ingo
Molnar in commit e58aa3d2 (genirq: Run irq
handlers with interrupts disabled), running IRQ handlers with interrupts
enabled can cause stack overflows when the interrupt line of the issuing
device is still active.

This patch removes IRQF_DISABLED from this documentation.  It was
mentioned to be a solution to avoid deadlocks when a device uses
multiple interrupts.  As the flag is a NOOP this solution does not work
anymore.
Signed-off-by: NValentin Rothberg <Valentin.Rothberg@lip6.fr>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/PCI/MSI-HOWTO.txt

@@ -501,18 +501,9 @@ necessary to disable interrupts (Linux guarantees the same interrupt will
 not be re-entered).  If a device uses multiple interrupts, the driver
 must disable interrupts while the lock is held.  If the device sends
 a different interrupt, the driver will deadlock trying to recursively
-acquire the spinlock.
-
-There are two solutions.  The first is to take the lock with
-spin_lock_irqsave() or spin_lock_irq() (see
-Documentation/DocBook/kernel-locking).  The second is to specify
-IRQF_DISABLED to request_irq() so that the kernel runs the entire
-interrupt routine with interrupts disabled.
-
-If your MSI interrupt routine does not hold the lock for the whole time
-it is running, the first solution may be best.  The second solution is
-normally preferred as it avoids making two transitions from interrupt
-disabled to enabled and back again.
+acquire the spinlock.  Such deadlocks can be avoided by using
+spin_lock_irqsave() or spin_lock_irq() which disable local interrupts
+and acquire the lock (see Documentation/DocBook/kernel-locking).
 
 4.6 How to tell whether MSI/MSI-X is enabled on a device