bpf: Add copy_map_value_long to copy to remote percpu memory

bpf_long_memcpy is used while copying to remote percpu regions from BPF
syscall and helpers, so that the copy is atomic at word size
granularity.

This might not be possible when you copy from map value hosting kptrs
from or to percpu maps, as the alignment or size in disjoint regions may
not be multiple of word size.

Hence, to avoid complicating the copy loop, we only use bpf_long_memcpy
when special fields are not present, otherwise use normal memcpy to copy
the disjoint regions.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220904204145.3089-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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