nfsd: fix error handling in nfs4_set_delegation()

I noticed a memory corruption crash in nfsd in
4.17-rc1. This patch corrects the issue.

Fix to return error if the delegation couldn't be hashed or there was
a recall in progress. Use the existing error path instead of
destroy_delegation() for readability.

Signed-off-by: Andrew Elble <aweits@rit.edu>
Fixes: 353601e7

 ("nfsd: create a separate lease for each delegation")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>