NFS: Do some tidying of the parsing code

Do some tidying of the parsing code, including:

 (*) Returning 0/error rather than true/false.

 (*) Putting the nfs_fs_context pointer first in some arg lists.

 (*) Unwrap some lines that will now fit on one line.

 (*) Provide unioned sockaddr/sockaddr_storage fields to avoid casts.

 (*) nfs_parse_devname() can paste its return values directly into the
     nfs_fs_context struct as that's where the caller puts them.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>