from32to16() is used by lib/checksum.c and also by
arch/parisc/lib/checksum.c. The next patch will use it in the
bpf_csum_diff helper.
Move from32to16() to the include/net/checksum.h as csum_from32to16() and
remove other implementations.
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>