illumos gate

jimmyH from the ZFS on Linux project recently pointed out a memory leak in the l2arc_write_done code. The memory leak happens if we compressed a buffer and so b_tmp_cdata contains a memory buffer holding the compressed buffer representation, but if for reason the ARC hash lock acquisition in l2arc_write_done misses out, we won't ever release this. Release of the compressed buffer should be done prior to hash lock acquisition, since reading the contents of b_l2hdr is always safe for us (because the buffer had been scheduled for L2ARC writing).