LU-18942 obdclass: rework limits for zfs

LU-18942 obdclass: rework limits for zfs

ZFS ARC is uncontrolled by Linux memory, so this size should not be
accounted for all caches, not just the lu_object cache. Let's reduce
the number of objects freed in a single batch to avoid high CPU usage
in ARC prune threads and increase latency in providing free space.

Test-Parameters: trivial
HPE-bug-id: LUS-12814, LUS-12813
Fixes: 79b4ae9139c ("LU-1305 osd: osd_handler")
Fixes: 0123baecc4e ("LU-5164 osd: Limit lu_object cache")
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: I5342149b185c61c56087d970f26eb4f197a597ef