bpf: unconditionally reset backtrack_state masks on global func exit

In mark_chain_precision() logic, when we reach the entry to a global
func, it is expected that R1-R5 might be still requested to be marked
precise. This would correspond to some integer input arguments being
tracked as precise. This is all expected and handled as a special case.

What's not expected is that we'll leave backtrack_state structure with
some register bits set. This is because for subsequent precision
propagations backtrack_state is reused without clearing masks, as all
code paths are carefully written in a way to leave empty backtrack_state
with zeroed out masks, for speed.

The fix is trivial, we always clear register bit in the register mask, and
then, optionally, set reg->precise if register is SCALAR_VALUE type.

Reported-by: Chris Mason <[email protected]>
Fixes: be2ef8161572 ("bpf: allow precision tracking for programs with subprogs")
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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