NFSD: remove unneeded linux/user_namespace.h include

After commit 4c1e1b34 ("nfsd: Store ex_anon_uid and ex_anon_gid as
kuids and kgids") using kuid/kgid for ex_anon_uid/ex_anon_gid,
user_namespace.h is not needed.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>