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 e31e9eb)

src/rgw/rgw_file.cc

@@ -1284,7 +1284,7 @@ int rgw_lookup(struct rgw_fs *rgw_fs,
 	return -ENOENT;
     }
   } else {
-    fhr = fs->stat_leaf(parent, path, RGWFileHandle::FLAG_EXACT_MATCH);
+    fhr = fs->stat_leaf(parent, path, RGWFileHandle::FLAG_NONE);
     if (! get<0>(fhr)) {
       if (! (flags & RGW_LOOKUP_FLAG_CREATE))
 	return -ENOENT;