Fix unbalanced mutex in dma_pool_create().

dma_pool_create() needs to unlock the mutex in error case.  The bug was
introduced in the 3.16 by commit cc6b664a ("mm/dmapool.c: remove
redundant NULL check for dev in dma_pool_create()")/

Signed-off-by: Krzysztof Hałasa <khc@piap.pl>
Cc: stable@vger.kernel.org  # v3.16
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>