fuse: Fix getxattr, listxattr calls and add a regression test (RHBZ#691389).

fuse: Fix getxattr, listxattr calls and add a regression test (RHBZ#691389).

The documentation for the getxattr and listxattr calls is not very
clear and as a result we were always returning something different
from that which the Linux kernel would usually return.

This fixes these calls, at least far enough that both the 'getfattr'
and 'getfacl' programs now work fine on FUSE-mounted filesystems.

Note that SELinux attrs are *not* passed through.  This appears to be
a known bug between SELinux and FUSE.  For more information see:

http://www.spinics.net/lists/selinux/msg09460.html