ptlrpc_request_alloc() takes an import reference (class_import_get()) and increments
imp_reqs. ptlrpc_request_free() drops neither. If a caller frees the request before
ptlrpc_request_pack(), both leak. The import is then never destroyed and never goes idle.

LU-20050 and LU-20546 fixed the sepol and pack-failure cases. Some pre-pack exits in
osc_brw_prep_request() still leak:

  - the encryption folio array allocation, obd_pool_get_folios_array(), and the per-page
    encryption failure
  - cl_env_get() failure
  - cl_object_attr_get() failure
  - the -ENODATA exit for an empty RPC to an old server

Possible fix: when rq_import is still set, let ptlrpc_request_free() itself decrement
imp_reqs and call class_import_put(). After LU-20546, a pack failure clears rq_import, so
callers that free after a failed pack stay correct. This needs an audit of all
ptlrpc_request_free() callers. It would also remove the open-coded put in
ptlrpc_request_alloc_internal().
  
Found during review of https://review.whamcloud.com/68582 (LU-20546)