NFSv4: Fix a memory leak bug

In nfs4_try_migration(), if nfs4_begin_drain_session() fails, the
previously allocated 'page' and 'locations' are not deallocated, leading to
memory leaks. To fix this issue, go to the 'out' label to free 'page' and
'locations' before returning the error.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>