common/ceph_time: tolerate mono time going backwards

common/ceph_time: tolerate mono time going backwards

Some kernels (and possibly some hardware?) can trigger a monotonic clock
that goes back in time.  That, in turn, can lead to a negative monotonic
time span.  This would trigger an assert.

This this problem seems to be widespread, tolerate the case and interpret
it as a 0-length interval (vs something negative).

Fixes: https://tracker.ceph.com/issues/44078
Fixes: https://tracker.ceph.com/issues/43365
Signed-off-by: Sage Weil <sage@redhat.com>