nfsd: consider CLAIM_FH when handing out delegation

CLAIM_FH was added by NFSv4.1.  It is the same as CLAIM_NULL except that it
uses only current FH to identify the file to be opened.

The NFS client is using CLAIM_FH if the FH is available when opening a file.
Currently, we cannot get any delegation if we stat a file before open it
because the server delegation code does not recognize CLAIM_FH.

We tested this patch and found delegation can be handed out now when claim is
CLAIM_FH.

See http://marc.info/?l=linux-nfs&m=136369847801388&w=2 and
http://www.linux-nfs.org/wiki/index.php/Server_4.0_and_4.1_issues#New_open_claim_types

Signed-off-by: Ming Chen <mchen@cs.stonybrook.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>