ubifs: fix build warning after debugfs cleanup patch

Stephen writes:
	After merging the driver-core tree, today's linux-next build (arm
	multi_v7_defconfig) produced this warning:

	fs/ubifs/debug.c: In function 'dbg_debugfs_init_fs':
	fs/ubifs/debug.c:2812:6: warning: unused variable 'err' [-Wunused-variable]
	  int err, n;
	      ^~~

So fix this up properly.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Richard Weinberger <richard@nod.at>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>