ceph: dump granular cap info in "caps" debugfs file

We have a "caps" file already that gives statistics on the caps
cache as a whole. Add another section to that output and dump a
line for each individual cap record.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>