bpf: Allow initializing dynptrs in kfuncs

bpf: Allow initializing dynptrs in kfuncs

This change allows kfuncs to take in an uninitialized dynptr as a
parameter. Before this change, only helper functions could successfully
use uninitialized dynptrs. This change moves the memory access check
(including stack state growing and slot marking) into
process_dynptr_func(), which both helpers and kfuncs call into.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230301154953.641654-4-joannelkoong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>