drivers: lustre: obdclass: check result of register_shrinker()

lu_global_init() does not check result of register_shrinker()
which was tagged __must_check recently, reported by sparse.
Patch also fixes missed cleanup of resources allocated prior to
register_shrinker() invocation and not freed after any failure.

Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>