NFSD: Cleanup unneeded including net/ipv6.h

Commit 49b28684 ("nfsd: Remove deprecated nfsctl system call and
related code") removed the only use of ipv6_addr_set_v4mapped(), so
net/ipv6.h is unneeded now.

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