test/client: zero-initialize getxattrv

since ll_getxattr() returns a non null terminated buffer -- the comparsion
gets polluted with garbage value like:

2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:./src/test/client/nonblocking.cc:1211: Failure
2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:Expected equality of these values:
2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:  setxattrv
2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:    Which is: "8388608"
2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:  getxattrv
2026-07-09T12:47:03.740 INFO:tasks.workunit.client.0.trial043.stdout:    Which is: "8388608R"

Fixes: https://tracker.ceph.com/issues/78126
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>