megaraid_sas: fix memory leak if SGL has zero length entries

commit 7a6a731bd00ca90d0e250867c3b9c05b5ff0fa49 upstream.

commit 98cb7e44 ([SCSI] megaraid_sas: Sanity check user
supplied length before passing it to dma_alloc_coherent())
introduced a memory leak.  Memory allocated for entries
following zero length SGL entries will not be freed.

Reference: http://bugs.debian.org/688198

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>