add struct file_ops for file type socket

add struct file_ops for file type socket

Even with some special handling needed in select_poll(), add a struct
file_ops for FTYPE_SOCKET. Due to the need of the special handling it
isn't possible to use a dynamically allocated file type.

Most functions calling the file_ops methods can be simplified a lot now
that no file type specific handling is left. Same applies to the file
type name printing in debug/verbose mode.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>