UBIFS: switch to dynamic printks

Switch to debugging using dynamic printk (pr_debug()). There is no good reason
to carry custom debugging prints if there is so cool and powerful generic
dynamic printk infrastructure, see Documentation/dynamic-debug-howto.txt. With
dynamic printks we can switch on/of individual prints, per-file, per-function
and per format messages. This means that instead of doing old-fashioned

echo 1 > /sys/module/ubifs/parameters/debug_msgs

to enable general messages, we can do:

echo 'format "UBIFS DBG gen" +ptlf' > control

to enable general messages and additionally ask the dynamic printk
infrastructure to print process ID, line number and function name. So there is
no reason to keep UBIFS-specific crud if there is more powerful generic thing.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>