ceph: check mapping to determine if FILE_CACHE cap is used

See if the i_data mapping has any pages to determine if the FILE_CACHE
capability is currently in use, instead of assuming it is any time the
rdcache_gen value is set (i.e., issued -> used).

This allows the MDS RECALL_STATE process work for inodes that have cached
pages.

Signed-off-by: Sage Weil <sage@newdream.net>