NFSv4.2: Fix up an invalid combination of memory allocation flags

We should use either GFP_KERNEL or GFP_NOFS, but not both. Also strip
GFP_KERNEL_ACCOUNT down to GFP_KERNEL. This memory is shrinkable, so
does not need to be limited by kmemcg.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>