failed in dbg_snprintf_key at 137 (pid 1)

This is a regression caused by "515315a UBIFS: fix key printing".

Fix the issue by increasing the buffer to 48 bytes.

Reported-by: Michael Hench <michaelhench@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Tested-by: Michael Hench <michaelhench@gmail.com>
Cc: stable@vger.kernel.org [v3.3+]
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