xen/irq: Alter the locking to use a mutex instead of a spinlock.

      When we allocate/change the IRQ informations, we do not
need to use spinlocks. We can use a mutex (which is
what the generic IRQ code does for allocations/changes).
Fixes a slew of:

BUG: sleeping function called from invalid context at /linux/kernel/mutex.c:271
in_atomic(): 1, irqs_disabled(): 0, pid: 3216, name: xenstored
2 locks held by xenstored/3216:
 #0:  (&u->bind_mutex){......}, at: [<ffffffffa02e0920>] evtchn_ioctl+0x30/0x3a0 [xen_evtchn]
 #1:  (irq_mapping_update_lock){......}, at: [<ffffffff8138b274>] bind_evtchn_to_irq+0x24/0x90
Pid: 3216, comm: xenstored Not tainted 3.1.0-rc6-00021-g437a3d1 #2
Call Trace:
 [<ffffffff81088d10>] __might_sleep+0x100/0x130
 [<ffffffff81645c2f>] mutex_lock_nested+0x2f/0x50
 [<ffffffff81627529>] __irq_alloc_descs+0x49/0x200
 [<ffffffffa02e0920>] ? evtchn_ioctl+0x30/0x3a0 [xen_evtchn]
 [<ffffffff8138b214>] xen_allocate_irq_dynamic+0x34/0x70
 [<ffffffff8138b2ad>] bind_evtchn_to_irq+0