mm/dmapool.c: take lock only once in dma_pool_free()

dma_pool_free() scans for the page to free in the pool list holding the
pool lock.  Then it releases the lock basically to acquire it immediately
again.  Modify the code to only take the lock once.

This will do some additional loops and computations with the lock held in
if memory debugging is activated.  If it is not activated the only new
operations with this lock is one if and one substraction.

Signed-off-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>