Fix unbalanced mutex in dma_pool_create().

commit 153a9f13 upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>