random: add and use memzero_explicit() for clearing data

random: add and use memzero_explicit() for clearing data

zatimend has reported that in his environment (3.16/gcc4.8.3/corei7)
memset() calls which clear out sensitive data in extract_{buf,entropy,
entropy_user}() in random driver are being optimized away by gcc.

Add a helper memzero_explicit() (similarly as explicit_bzero() variants)
that can be used in such cases where a variable with sensitive data is
being cleared out in the end. Other use cases might also be in crypto
code. [ I have put this into lib/string.c though, as it's always built-in
and doesn't need any dependencies then. ]

Fixes kernel bugzilla: 82041

Reported-by: [email protected]
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Hannes Frederic Sowa <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>
Cc: [email protected]