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 <lilingfeng3@huawei.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

fs/nfs/getroot.c

@@ -62,7 +62,7 @@ static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *i
 }
 
 /*
- * get an NFS2/NFS3 root dentry from the root filehandle
+ * get a root dentry from the root filehandle
  */
 int nfs_get_root(struct super_block *s, struct fs_context *fc)
 {