Tools: hv: vssdaemon: ignore the EBUSY on multiple freezing the same partition

If a partition appears mounted more than once in /proc/mounts, vss_do_freeze()
succeeds only for the first time and gets EBUSY (on freeze) or EINVAL (on
thaw) for the second time. The patch ignores these to make the backup feature
work.

Also improved the error handling in case a freeze operation fails.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>