genalloc: fix allocation from end of pool

bitmap_find_next_zero_area requires the size of the bitmap, we instead
passed the last suitable position.  This made it impossible to allocate
from the end of the pool.

Fixes a regression introduced by 243797f5
("genalloc: use bitmap_find_next_zero_area").

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Cc: Zygo Blaxell <zygo.blaxell@xandros.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>