common: OutputDataSocket retakes mutex on error path

common: OutputDataSocket retakes mutex on error path

the list of buffers is protected by mutex m_lock. when dump_data() fails
and puts unwritten buffers back into the list, it needs to reaquire the
lock

Fixes: http://tracker.ceph.com/issues/40188
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 8645e24dd4fb3ecc9f02b34a1be916620b5f71b1)

Conflicts:
    src/common/OutputDataSocket.cc
- use Mutex::Locker instead of std::scoped_lock