fs: fix symlink error message

the arguments were swapped, so fs.symlink{Sync,} would
report that the wrong file EEXIST'd in error.

Fixes: https://github.com/joyent/node/issues/8651
Fixes: https://github.com/joyent/node/issues/4314
Fixes: https://github.com/joyent/node/issues/5381
PR-URL: https://github.com/joyent/node/pull/8657
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>