UBIFS: make debugging messages light again

UBIFS: make debugging messages light again

We switch to dynamic debugging in commit
dee87f8fc5116ea85d735696c88b0002dea87c53 but did not take into account that
now we do not control anymore whether a specific message is enabled or not.
So now we lock the "dbg_lock" and release it in every debugging macro, which
make them not so light-weight.

This commit removes the "dbg_lock" protection from the debugging macros to
fix the issue.

The downside is that now our DBGKEY() stuff is broken, but this is not
critical at all and will be fixed later.

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