EX-8270 ptlrpc: rename 'size_bits' to 'order'

EX-8270 ptlrpc: rename 'size_bits' to 'order'

The kernel uses 'order' to refer to page allocations of a
certain 'order', meaning 2^order pages.

That's what our 'size bits' is - an allocation of a certain
'order'.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I38b184239814a0f692b644566075c798ed16f816
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52427
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>