NFSv4: Directly use ida_alloc()/free()

Use ida_alloc()/ida_free() instead of
ida_simple_get()/ida_simple_remove().
The latter is deprecated and more verbose.

Signed-off-by: Bo Liu <liubo03@inspur.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>