NFSD: Refactor nfsd4_cleanup_inter_ssc() (1/2)

The @src parameter is sometimes a pointer to a struct nfsd_file and
sometimes a pointer to struct file hiding in a phony struct
nfsd_file. Refactor nfsd4_cleanup_inter_ssc() so the @src parameter
is always an explicit struct file.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>