bpf: Check the other end of slot_type for STACK_SPILL

Every 8 bytes of the stack is tracked by a bpf_stack_state.
Within each bpf_stack_state, there is a 'u8 slot_type[8]' to track
the type of each byte.  Verifier tests slot_type[0] == STACK_SPILL
to decide if the spilled reg state is saved.  Verifier currently only
saves the reg state if the whole 8 bytes are spilled to the stack,
so checking the slot_type[7] is the same as checking slot_type[0].

The later patch will allow verifier to save the bounded scalar
reg also for <8 bytes spill.  There is a llvm patch [1] to ensure
the <8 bytes spill will be 8-byte aligned,  so checking
slot_type[7] instead of slot_type[0] is required.

While at it, this patch refactors the slot_type[0] == STACK_SPILL
test into a new function is_spilled_reg() and change the
slot_type[0] check to slot_type[7] check in there also.

[1] https://reviews.llvm.org/D109073

Signed-off-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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