Windows: cm_BPlusDirBuildTree can fail

Windows: cm_BPlusDirBuildTree can fail

It is possible that cm_BPlusDirBuildTree can fail.  For example,
the server could be marked down after a callback is obtained
but before all of the directory data buffers have been fetched.
cm_BeginDirOp must check for the failure, destroy the tree,
and return the failure code to the caller.  Otherwise, a tree
with no entries may be created and marked with the current
data version.

LICENSE MIT

Change-Id: I26fbfceaf68389a1906797b12721c49172b027ec
Reviewed-on: http://gerrit.openafs.org/893
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>