UBIFS: fix debugging messages

Patch 56e46742 broke UBIFS debugging messages:
before that commit when UBIFS debugging was enabled, users saw few useful
debugging messages after mount. However, that patch turned 'dbg_msg()' into
'pr_debug()', so to enable the debugging messages users have to enable them
first via /sys/kernel/debug/dynamic_debug/control, which is very impractical.

This commit makes 'dbg_msg()' to use 'printk()' instead of 'pr_debug()', just
as it was before the breakage.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@kernel.org [3.0+]