LU-13359 quota: call rhashtable_lookup near params decl

LU-13359 quota: call rhashtable_lookup near params decl

rhashtable_lookup() is an inline function which depends - for
performancs - on the 'rhashtable_params' being visible and
consnt.  So it should only be called in the same file that
declared the params.

A recent patch make pools_hash_params an external variable and calls
rhashtable_lookup from a separate file, which will break the
optimisation.

So add lov_pool_find() and use it to maintainer optimization.

Fixes: 6b9f849fd5f4 ("LU-13359 quota: make used for pool correct")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ieeb3b080491f5b2c9c825885fe7a42f4a8599a2a
Reviewed-on: https://review.whamcloud.com/39676
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>