Filehandle lookup currently breaks out the interesting pieces of
a filehandle and passes them to exp_get or exp_get_fsid, which put the
pieces back into a filehandle fragment.

We define a new interface "exp_find" which does a lookup based on
a filehandle fragment to avoid this double handling.

In the process, common code in exp_get_key and exp_get_fsid_key is united
into exp_find_key.

Also, filehandle composition now uses the mk_fsid_v? inline functions.