EX-8270 ptlrpc: replace ELEMENT_SIZE

EX-8270 ptlrpc: replace ELEMENT_SIZE

The ELEMENT_SIZE macro is fine, but it takes a pool index
and doesn't handle the pool of order 0.  Change it to a
function.  (This is marginally less efficient in one spot,
since it replaces a shift with a divide, but it should be
just fine.)

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I322037e50bbdb8e0274b37f82618b6907b6d2906
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52445
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>