fix nfs_ioctl() so that it does not use v_mount when the vnode is unlocked

[rmacklem]

Summary

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().

Test Plan

Tested for NFSv3 and NFSv4.2 mounts (without and
with a Seek operation).