hpet: kill BKL, add compat_ioctl

hpet uses the big kernel lock in its ioctl and open
functions. Replace this with a private mutex to be
sure. Since we're already touching the ioctl function,
add the compat_ioctl version as well -- all commands
except HPET_INFO are compatible and that one is easy
to add.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: Bob Picco <bob.picco@hp.com>