LU-12064 ptlrpc: set at_min=5 by default

LU-12064 ptlrpc: set at_min=5 by default

Having at_min=0 as the default value can result in clients timing
out and/or being evicted too easily when there is a sudden spike
in server load.  Increase at_min to 5s by default.

For large clusters, at_min=15 is more reasonable, but distributing
a variable at_min value to clients will need more complex changes.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3463cbc642458f6dd5977fe34478b135d1cd0219
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50609
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>