ubifs: Fix regression in ubifs_readdir()

Commit c83ed4c9 ("ubifs: Abort readdir upon error") broke
overlayfs support because the fix exposed an internal error
code to VFS.

Reported-by: Peter Rosin <peda@axentia.se>
Tested-by: Peter Rosin <peda@axentia.se>
Reported-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Tested-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Fixes: c83ed4c9 ("ubifs: Abort readdir upon error")
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>