autofs4: fix symlink name allocation

The length of the symlink name has been moved but it needs to be set
before allocating space for it in the dentry info struct.  This corrects a
mistake in a recent patch.

Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>