We need this helper to skip over special fields (bpf_spin_lock,
bpf_timer, kptrs) while zeroing a map value. Use the same logic as
copy_map_value but memset instead of memcpy.

Currently, the code zeroing map value memory does not have to deal with
special fields, hence this is a prerequisite for introducing such
support.
Signed-off-by: NKumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220904204145.3089-4-memxor@gmail.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>