cifs: Recognize SFU socket type

SFU since its (first) version 3.0 supports AF_LOCAL sockets and stores them
on filesytem as system file with one zero byte. Add support for detecting
this SFU socket type into cifs_sfu_type() function.

With this change cifs_sfu_type() would correctly detect all special file
types created by SFU: fifo, socket, symlink, block and char.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>