bpf: Explicitly memset the bpf_attr structure

For the bpf syscall, we are relying on the compiler to properly zero out
the bpf_attr union that we copy userspace data into. Unfortunately that
doesn't always work properly, padding and other oddities might not be
correctly zeroed, and in some tests odd things have been found when the
stack is pre-initialized to other values.

Fix this by explicitly memsetting the structure to 0 before using it.

Reported-by: Maciej Żenczykowski <[email protected]>
Reported-by: John Stultz <[email protected]>
Reported-by: Alexander Potapenko <[email protected]>
Reported-by: Alistair Delva <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://android-review.googlesource.com/c/kernel/common/+/1235490
Link: https://lore.kernel.org/bpf/[email protected]

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