reiserfs: rework priv inode handling

Reiserfs is the only filesystem that removes IOP_XATTR without also
using a set of dedicated inode operations at the same time that nop all
xattr related inode operations. This means we need to have a IOP_XATTR
check in vfs_listxattr() instead of just being able to check for
->listxatt() being implemented.

Introduce a dedicated set of nop inode operations that are used when
IOP_XATTR is removed, allowing us to remove that check from
vfs_listxattr(). This in turn allows us to completely decouple POSIX ACLs from
IOP_XATTR.

Cc: reiserfs-devel@vger.kernel.org
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>