cls/user: reset stats only returns marker when truncated

cls/user: reset stats only returns marker when truncated

the returned marker is a bucket name. when bucket names are long, the
response can overflow the 64-byte limit on responses to write operations
with librados::OPERATION_RETURNVEC. this leads to errors like:

> ERROR: could not reset user stats: (75) Value too large for defined data type

however, the client only needs this marker string to resume listing
after a truncated response. if the listing is not truncated, we can omit
the marker to save space

in general, users will have less than MAX_ENTRIES=1000 buckets, so won't
get truncated listings

Fixes: https://tracker.ceph.com/issues/51786
Signed-off-by: Casey Bodley <cbodley@redhat.com>