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().

(cherry picked from commit bffb3d947b2ee48dc30c4876e0c7a5927375e38f)