rgw_file: fix hiwat behavior

rgw_file: fix hiwat behavior

Removed logic to skip reclaim processing conditionally on hiwat,
this probably meant to be related to a lowat value, which does
not exist.

Having exercised the hiwat reclaim behavior, noticed that the
path which moves unreachable objects to LRU, could and probably
should remove them altogether when q.size exceeds hiwat.  Now
the max unreachable float is lane hiwat, for all lanes.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>