nfs_clvnops.c: Fix access to v_mount when vnode unlocked

Commit ab17854f974b fixed access to v_mount when the
vnode is unlocked for nfs_copy_file_range().

This patch does the same for nfs_ioctl().

Reviewed by: kib, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36846