LU-17422 obdclass: rename sptlrpc pool and move init

LU-17422 obdclass: rename sptlrpc pool and move init

This patch completes the move of the pools code to obd by
renaming the sptlrpc pool to obd, and moves the pool init
and cleanup to obd.

Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: I9164601745c8faf19559216f55ea5df4e2e226fe
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53669
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>