signal: Correct the offset of si_pkey in struct siginfo

The change moving addr_lsb into the _sigfault union failed to take
into account that _sigfault._addr_bnd._lower being a pointer forced
the entire union to have pointer alignment.  In practice this only
mattered for the offset of si_pkey which is why this has taken so long
to discover.

To correct this change _dummy_pkey and _dummy_bnd to have pointer type.

Reported-by: kernel test robot <shun.hao@intel.com>
Fixes: b68a68d3

 ("signal: Move addr_lsb into the _sigfault union for clarity")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>