UBIFS: fix key printing

Before commit 56e46742 we have had locking
around all printing macros and we could use static buffers for creating
key strings and printing them. However, now we do not have that locking and
we cannot use static buffers. This commit removes the old DBGKEY() macros
and introduces few new helper macros for printing debugging messages plus
a key at the end. Thankfully, all the messages are already structures in
a way that the key is printed in the end.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>