chain_xattr: s/onechunk/skip_chain_cleanup, add ensure_single_attr

chain_xattr: s/onechunk/skip_chain_cleanup, add ensure_single_attr

onechunk was a deceptive name since it didn't actually ensure that
a single chunk would be used.  Rename to ensure_single_attr.  Also,
add a parameter to ensure that we use a single attribute.  We need
these to be distinct since we have LFN attrs which have been split
over 254 byte xattrs since after hammer which we need to correctly
clean up.  However, we need to ensure going forward that those attrs
are never split over more than one chunk -- it's not atomic.
Further, skip_chain_cleanup should imply ensure_single_attr, so make
them template params and add a static check to make sure we don't
mess this up in the future.

Signed-off-by: Samuel Just <sjust@redhat.com>