LU-17025 llapi: restore 'pool=ignore' functionality

LU-17025 llapi: restore 'pool=ignore' functionality

Changes to llapi_stripe_param_verify() and related llapi file
creation functions to verify that the given pool name is valid
introduced a bug that disallowed the 'ignore' pool name, which
is used to create files without any pool name.

Allow the reserved pool names from lov_pool_is_reserved() to be
used even (especially!) if the named pool does not exist.

Revert the changes to ost-pools.sh::test_32() that created the
'ignore_pool' pool, and go back to checking that 'ignore' will
create a file that does not use any pool.

Change the pool name validation to only do fsname lookup if the
pool name is actually specified, instead of looking up fsname
but not actually using it for anything.

Fixes: ee7dfc5ad1 ("LU-17025 llapi: Verify stripe pool name")
Signed-off-by: Rajeev Mishra <rajeevm@hpe.com>
Change-Id: I9368f28a41fd9af6b6f0e9468df0e7dfd728db1c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54355
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>