Windows: do not leak scp->dirlock if cm_BPlusDirBuildTree fails

Windows: do not leak scp->dirlock if cm_BPlusDirBuildTree fails

In cm_BeginDirOp, the scp->dirlock would be leaked if
cm_BPlusDirBuildTree() failed.   This would either result in
a panic later on if lock order validation is active; or as
an inability to process subsequent requests on the directory.

LICENSE MIT

Change-Id: I03afd0c9e6296c0f43ae39e5a7b1ff29a1619a43
Reviewed-on: http://gerrit.openafs.org/1083
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>