nfs: fix the comment of nfs_get_root

nfs: fix the comment of nfs_get_root

The comment for nfs_get_root() needs to be updated as it would also be
used by NFS4 as follows:
@x[
    nfs_get_root+1
    nfs_get_tree_common+1819
    nfs_get_tree+2594
    vfs_get_tree+73
    fc_mount+23
    do_nfs4_mount+498
    nfs4_try_get_tree+134
    nfs_get_tree+2562
    vfs_get_tree+73
    path_mount+2776
    do_mount+226
    __se_sys_mount+343
    __x64_sys_mount+106
    do_syscall_64+69
    entry_SYSCALL_64_after_hwframe+97
, mount.nfs4]: 1

Signed-off-by: Li Lingfeng <[email protected]>
Acked-by: Jeff Layton <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>