Add a check that the user-provided option is at least as long as the
number of bytes we intend to read. Before this patch we would blindly
read sizeof(int) bytes even in cases where the user passed
optlen<sizeof(int), which would potentially read garbage or fault.

Discovered by new tests in https://github.com/google/gvisor/pull/6957 .

The original get_user call predates history in the git repo.
Signed-off-by: NTamir Duberstein <tamird@gmail.com>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20211229200947.2862255-1-willemdebruijn.kernel@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>