um: siginfo cleanup

Currently we use both struct siginfo and siginfo_t.
Let's use struct siginfo internally to avoid ongoing
compiler warning. We are allowed to do so because
struct siginfo and siginfo_t are equivalent.

[cherry-pick of upstream 9a8c1359571c5d5e2fbc43cf457a6486b70a70cb]

Change-Id: I564775c9ed515c39ffff7d7d600a85d50291f31d
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>