mds: avoid spurious sleeps

mds: avoid spurious sleeps

Like trim_interval, don't sleep for small amounts of time. This avoids
spurious sleeps like:

    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s
    2020-12-25T00:14:22.242+0000 7f6a95884700 20 mds.0.cache upkeep thread waiting interval 0.000000108s

Also, fix the same issue in the Client.

Fixes: https://tracker.ceph.com/issues/48753
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>