scsi: storvsc: Fix a bug in copy_from_bounce_buffer()

We may exit this function without properly freeing up the maapings
we may have acquired. Fix the bug.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: James Bottomley <JBottomley@Odin.com>