NFS: Add missing null check for failed allocation

Currently the allocation of buf is not being null checked and
a null pointer dereference can occur when the memory allocation fails.
Fix this by adding a check and returning -ENOMEM.

Addresses-Coverity: ("Dereference null return")
Fixes: 6d972518b821 ("NFS: Add fs_context support.")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>