cephfs: add auth caps based on fs names

cephfs: add auth caps based on fs names

Add new auth caps to restrict access to clients based on fsnames. To
specify this, for example:

mds 'allow rw fsname=cephfs1'

This will restrict client access to fs name "cephfs1" only. Messages to
active MDS assigned to any other FSMap will be dropped. Standby MDS not
associated with an FSMap will accept messages from clients. To allow
multiple file systems, create MDS cap as follows -

mds 'allow rw fsname=cephfs1, allow rw fsname=cephfs2'

Fixes: http://tracker.ceph.com/issues/15070
Signed-off-by: Douglas Fuller <dfuller@redhat.com>
Signed-off-by: Rishabh Dave <ridave@redhat.com>