crimson/monc: honor auth_result_t::canceled as the result of do_auth().

crimson/monc: honor auth_result_t::canceled as the result of do_auth().

An attempt to `Connection::do_auth()` may finish in one of three states:
_success_, _failure_ and _cancellation_. Unfortunately, its callers were
missing the third treating cancellation like a failure. This was the root
cause of the following failure at Sepia:

```
rzarzynski@teuthology:/home/teuthworker/archive/rzarzynski-2021-05-06_22:08:43-rados-master-distro-basic-smithi/6102605$ less ./remote/smithi204/log/ceph-osd.3.log.gz
...
WARN  2021-05-06 22:35:40,464 [shard 0] osd - ms_handle_reset
...
INFO  2021-05-06 22:35:40,465 [shard 0] monc - do_auth_single: connection closed
INFO  2021-05-06 22:35:40,465 [shard 0] ms - [osd.3(client) v2:172.21.15.204:6808/31418@57568 >> mon.? v2:172.21.15.204:3300/0] execute_connecting(): protocol aborted at CLOSING -- std::system_error (error crimson::net:6, protocol aborted)
...
ERROR 2021-05-06 22:35:40,465 [shard 0] osd - mon.osd.3 dispatch() ms_handle_reset caught exception: std::system_error (error crimson::net:3, negotiation failure)
ceph-osd: /home/jenkins-build/build/workspace/ceph-dev-new-build/ARCH/x86_64/AVAILABLE_ARCH/x86_64/AVAILABLE_DIST/centos8/DIST/centos8/MACHINE_SIZE/gigantic/release/17.0.0-3909-g81233a18/rpm/el8/BUILD/ceph-17.0.0-3909-g81233a18/src/crimson/common/gated.h:36: crimson::common::Gated::dispatch(const char*, T&, Func&&) [with Func = crimson::mon::Client::ms_handle_reset(crimson::net::ConnectionRef, bool)::<lambda()>&; T = crimson::mon::Client]::<lambda(std::__exception_ptr::exception_ptr)>: Assertion `*eptr.__cxa_exception_type() == typeid(seastar::gate_closed_exception)' failed.
Aborting on shard 0.
Backtrace:
 0# 0x00005618C973932F in ceph-osd
 1# FatalSignal::signaled(int, siginfo_t const*) in ceph-osd
 2# FatalSignal::install_oneshot_signal_handler<6>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) in ceph-osd
 3# 0x00007F7BB592EB20 in /lib64/libpthread.so.0
 4# gsignal in /lib64/libc.so.6
 5# abort in /lib64/libc.so.6
 6# 0x00007F7BB3F29B09 in /lib64/libc.so.6
 7# 0x00007F7BB3F37DE6 in /lib64/libc.so.6
 8# 0x00005618C9FF295C in ceph-osd
 9# 0x00005618C3907313 in ceph-osd
10# 0x00005618CCA2F84F in ceph-osd
11# 0x00005618CCA34D90 in ceph-osd
12# 0x00005618CCBEC9BB in ceph-osd
13# 0x00005618CC744E9A in ceph-osd
14# main in ceph-osd
15# __libc_start_main in /lib64/libc.so.6
16# _start in ceph-osd
daemon-helper: command crashed with signal 6
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>