mei: implement fsync

When write() returns successfully, it is only assumed that a message
was successfully queued. Add fsync syscall implementation to help
user-space ensure that all data is written.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>