NFSv4: Fixed max path length issue for symlinks.

Changes: Use MAXNAMLEN to MAXPATHLEN.

Steps to reproduce:
1. Mount the share with NFSv4;
2. Try to create symlink on the file with path more than 256 symbols;
3. Get the next error:

ln -s ./_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789/_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789 FILENAME

ln: creating symbolic link `FILENAME' -> `./_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789/_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789_123456789': File name too long

If the path is less than 256 symblols - all is OK.