checksum: Fix incorrect use of __le16/32/64

checksum: Fix incorrect use of __le16/32/64

Use ceph_le16/32/64 instead of __le16/32/64 (which are no-op outside
of kernel code).

Note that I've changed the Alg::calc routines to return the
init_value_t type instead of value_t, to avoid having to
introduce explicit byte-swapping calls to init_le16/32/64 in
many places.  (This way, the byte-swapping happens implicitly
at the very end, wher the init_value_t value is assigned via
a value_t pointer.)

Fixes (partially): https://tracker.ceph.com/issues/41605

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>