hv: hypervvssd: call endmntent before call setmntent again

If freeze fails, vss_operate will re-enter itself to thaw. But it forgets
to call endmntent() before it recalls setmntent() again.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>