NFSD enforce filehandle check for source file in COPY

If the passed in filehandle for the source file in the COPY operation
is not a regular file, the server MUST return NFS4ERR_WRONG_TYPE.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>