EX-8270 ptlrpc: simplify pool arrays

EX-8270 ptlrpc: simplify pool arrays

Currently, we do a fancy trick where we have a pool of
order 0, then subsequent pools start at
PPOOL_MIN_CHUNK_BITS (which is actually the minimum
compresison size).

So pool index 1 isn't a pool of order 1 (2 pages), it's a
pool of order PPOOL_MIN_CHUNK_BITS.

All this saves us is the cost of the empty pools below
PPOOL_MIN_CHUNK_BITS, but it makes the code notably harder
to read.

With this change, the order of the pool and the pool index
are the same.  This simplification will be embraced more
in subsequent patches.

Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I650e05d25727f10b0ca2d556cba17e9c4fccc309
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52452
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>