src/mds: allow passing fs names and path in same cap

src/mds: allow passing fs names and path in same cap

Allow passing path along with fs names and capspec while creating an MDS
cap. The new syntax looks as follows -

allow rw fsname=<fsname> path=<path>

To provide caps for multiple file systems, pass the same phrase multiple
times separated by commas -

allow rw fsname=<fsname1> path=<path1>, allow rw fsname=<fsname2>
path=<path2>, ...

This commit also makes sure that the old syntax 'allow rw path=<path>'
is supported for backwards compatibility. The old syntax would imply
'allow rw fsname=* path=<path>' and would grant read-write permission for
all FSs containing the path <path>.

Signed-off-by: Rishabh Dave <ridave@redhat.com>