do_timer must be called with xtime_lock held.  I'm not sure boot_timer_handler
needs this, however I don't think it hurts: it simply disables irq and takes a
spinlock.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>