hpet: factor timer allocate from open

hpet: factor timer allocate from open

The current implementation of the /dev/hpet driver couples opening the
device with allocating one of the (scarce) timers (aka comparators).  This
is a limitation in that the main counter may be valuable to applications
seeking a high-resolution timer who have no use for the interrupt
generating functionality of the comparators.

This patch alters the open semantics so that when the device is opened, no
timer is allocated.  Operations that depend on a timer being in context
implicitly attempt allocating a timer, to maintain backward compatibility.
 There is also an IOCTL (HPET_ALLOC_TIMER _IO) added so that the
allocation may be done explicitly.  (I prefer the explicit open then
allocate pattern but don't know how practical it would be to require all
existing code to be changed.)

/dev/hpet is accessed via mmap().  This is the only interface of /dev/hpet
that is actually used in practice.

[akpm@linux-foundation.org: coding-style tweaks]
[arnd@arndb.de: fix build]
Signed-off-by: Magnus Lynch <maglyx@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>