Factor out a small common piece of the path that allocate nfs_direct_req
structures.

Test plan:
Compile kernel with CONFIG_NFS and CONFIG_NFS_DIRECTIO enabled.
Signed-off-by: NChuck Lever <cel@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>