bpf: Distinguish fixed- and variable-size kfunc mem args with MEM_FIXED_SIZE

A kfunc memory-pointer argument comes in two flavors: a fixed-size buffer
whose access size is derived from the pointed-to BTF type, and a
variable-size buffer paired with a following __sz/__szk size argument.
Both were represented by separate kfunc_ptr_arg_type values
(KF_ARG_PTR_TO_MEM vs KF_ARG_PTR_TO_MEM_SIZE) with the pointer classified
as the latter when a size argument followed.

Mirror how helpers describe the same distinction: classify both as
KF_ARG_PTR_TO_MEM and OR in MEM_FIXED_SIZE for the fixed-size case, just
as helpers use ARG_PTR_TO_MEM | MEM_FIXED_SIZE. The switches now key on
base_type(kf_arg_type) so the flag rides along, and the KF_ARG_PTR_TO_MEM
handler either resolves the size from BTF (MEM_FIXED_SIZE) or falls
through to the mem/size-pair check, which validates the buffer against the
following size register and skips it. No functional change.

Currently, KF_ARG_MEM_SIZE and KF_ARG_CONST_MEM_SIZE are only reachable
from ARG_PTR_TO_MEM fallthrough. A patch later will merge scalar
checking into the same switch and remove the fallthrough.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260801074633.1595644-15-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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