ceph: don't include used caps in cap_wanted

when copying files to cephfs, file data may stay in page cache after
corresponding file is closed. Cached data use Fc capability. If we
include Fc capability in cap_wanted, MDS will treat files with cached
data as open files, and journal them in an EOpen event when trimming
log segment.

Signed-off-by: Yan, Zheng <zyan@redhat.com>