NFS: Remove unnecessary check in nfs_read_folio()

All the callers are expected to supply a valid struct file argument, so
there is no need for the NULL check.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>