bpf: Fold __szk const size handling into the scalar arg path

To align helper and kfunc pointer to memory argument handling, move
kfunc constant memorry size argument handling to the kfunc scalar
section. In addition, factor out constant scalar argument handling.

The constant size argument (__szk) of a kfunc memory/size pair was
recorded into meta->arg_constant by a dedicated block in the
KF_ARG_PTR_TO_MEM_SIZE case, duplicating the "only one constant
argument" and "must be a known constant" checks already in the generic
scalar argument handling. That block also did an explicit i++ to skip
the size argument.

This also fixes a precision gap: the old dedicated block did not mark
the size register precise, relying on check_mem_size_reg() for that. But
check_mem_size_reg() is skipped when the buffer is a nullable arg passed
as NULL (e.g. bpf_dynptr_slice(_rdwr) with a NULL buffer), so in that
case the __szk value was recorded and used for regs[R0].mem_size without
marking it precise. Routing the size through the scalar path marks it
precise in all cases.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260801074633.1595644-11-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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