compat_ioctl: block: simplify compat_blkpg_ioctl()

There is no need to go through a compat_alloc_user_space()
copy any more, just wrap the function in a small helper that
works the same way for native and compat mode.

Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>