bpf: make liveness.c track stack with 4-byte granularity

bpf: make liveness.c track stack with 4-byte granularity

Convert liveness bitmask type from u64 to spis_t, doubling the number
of trackable stack slots from 64 to 128 to support 4-byte granularity.

Each 8-byte SPI now maps to two consecutive 4-byte sub-slots in the
bitmask: spi*2 half and spi*2+1 half. In verifier.c,
check_stack_write_fixed_off() now reports 4-byte aligned writes of
4-byte writes as half-slot marks and 8-byte aligned 8-byte writes as
two slots. Similar logic applied in check_stack_read_fixed_off().

Queries (is_live_before) are not yet migrated to half-slot
granularity.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260410-patch-set-v4-4-5d4eecb343db@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>