bpf: Consolidate function call pkt_access validation

check_func_arg() checks whether a helper permits packet pointers before
dispatching argument-specific memory validation. check_kfunc_args() has
no equivalent check, even though kfunc memory arguments may be backed by
packet data.

Move packet-access validation to check_helper_mem_access(), where the
access direction is known and helper, kfunc, and global-subprogram memory
arguments converge. Pass call metadata there so helpers continue to
require bpf_func_proto::pkt_access, while writes through kfunc and global
subprogram arguments use the program-type policy and set
env->seen_direct_write.

Keep call metadata when variable-size memory disables raw mode by
clearing arg_raw_mem.regno instead, and pass it through the map-key path
as well.

This also makes kfunc and global-subprogram packet writes request the
required writable-packet prologue and rejects them for program types
that only support direct packet reads.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260911220415.1396439-20-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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