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 <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

fs/nfs/getroot.c

···
 }
 
 /*
- * 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)
 {