mds: ignore unknown client op when tracking op latency

mds: ignore unknown client op when tracking op latency

Server::handle_client_request() ignores unknown client operation
by returning -ENOTSUPP, however, Server::perf_gather_op_latency()
aborts on unknown client op, thereby causing -ENOTSUPP to never
reach the client. ceph_abort() seems unnecessary here.

Note, we could have invoked Server::perf_gather_op_latency()
when the return value to client is not -ENOTSUPP, however,
a valid client operation *might* just return -ENOTSUPP in
some cases.

@mchangir ran into this with his getvxattr op changes (PR #42001).

Signed-off-by: Venky Shankar <vshankar@redhat.com>