Fix arena_dalloc() performance regression.

Take into account large_pad when computing whether to pass the
deallocation request to tcache_dalloc_large(), so that the largest
cacheable size makes it back to tcache.  This regression was introduced
by 8a03cf039cd06f9fa6972711195055d865673966 (Implement cache index
randomization for large allocations.).

Bug: 21326736
(cherry picked from commit d24af195bf44844eb4baffbacd63100b8fe01472)

Change-Id: I9931df512e1aad623b76432b33046ab02a837866
1 file changed