rgw_file: invalid use of RGWFileHandle::FLAG_EXACT_MATCH

rgw_file: invalid use of RGWFileHandle::FLAG_EXACT_MATCH

The change which introduced this flag also caused it to be
given as the flags argument to RGWLibFS::stat_leaf() when called
from rgw_lookup().

This was incorrect:  in particular, when a directory is known only
as a common prefix of other objects, the AWS namespace mapping
convention requires lookup("foo") to match a non-materialized
instance of "foo/" (case 2 in RGWLibFS::stat_leaf's stat loop).

Fixes: http://tracker.ceph.com/issues/18992
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit e31e9eb980f958640150e8d7f17de1b9e5478b1e)