Windows: wait for I/O on buffers to complete in cm_SetupStoreBIOD

Windows: wait for I/O on buffers to complete in cm_SetupStoreBIOD

cm_SetupStoreBIOD constructs a list of dirty buffers for a file
that are to be written to the file server.  When constructing
the list, if when determining the first dirty buffer we come across
a buffer that is already actively involved in an I/O operation,
call buf_WaitIO() to wait until the buffer is no longer busy before
continuing.  This reduces lock contention and synchronization
conflicts.

LICENSE MIT

Change-Id: Icd79b9578b7411b138f14257bcb885249960eab0
Reviewed-on: http://gerrit.openafs.org/1760
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>