get rid of csum_partial_copy_to_user()

For historical reasons some architectures call their csum_and_copy_to_user()
csum_partial_copy_to_user() instead (and supply a macro defining the
former as the latter).  That's the last remnants of old experiment that
went nowhere; time to bury them.  Rename those to csum_and_copy_to_user()
and get rid of the macros.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>