nfsd: clean up nfs4_set_delegation

Move the alloc_init_deleg call into nfs4_set_delegation and change the
function to return a pointer to the delegation or an IS_ERR return. This
allows us to skip allocating a delegation if the file has already
experienced a lease conflict.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>