NFSD: forget_delegations should use list_for_each_entry_safe

Otherwise the for loop could try to use a file recently removed from the
file_hashtbl list and oops.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Tested-by: Casey Bodley <cbodley@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>