fuse: readdir: check for slash in names

commit efeb9e60d48f7778fdcad4a0f3ad9ea9b19e5dfd upstream.

Userspace can add names containing a slash character to the directory
listing.  Don't allow this as it could cause all sorts of trouble.

Change-Id: I7836dab7a5fedbf593f201e3a8506a94d93f4051
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
[bwh: Backported to 3.2: drop changes to parse_dirplusfile() which we
 don't have]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>