xattr: Fix setting security xattrs on sockfs

The IOP_XATTR flag is set on sockfs because sockfs supports getting the
"system.sockprotoname" xattr.  Since commit 6c6ef9f2

, this flag is checked for
setxattr support as well.  This is wrong on sockfs because security xattr
support there is supposed to be provided by security_inode_setsecurity.  The
smack security module relies on socket labels (xattrs).

Fix this by adding a security xattr handler on sockfs that returns
-EAGAIN, and by checking for -EAGAIN in setxattr.

We cannot simply check for -EOPNOTSUPP in setxattr because there are
filesystems that neither have direct security xattr support nor support
via security_inode_setsecurity.  A more proper fix might be to move the
call to security_inode_setsecurity into sockfs, but it's not clear to me
if that is safe: we would end up calling security_inode_post_setxattr after
that as well.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>