NFSv4.2: Fix a potential double free with READ_PLUS

kfree()-ing the scratch page isn't enough, we also need to set the pointer
back to NULL to avoid a double-free in the case of a resend.

Fixes: fbd2a05f

 (NFSv4.2: Rework scratch handling for READ_PLUS)
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>