vfs_lookup: Allow PATH_MAX-1 symlinks

Previously, symlinks in FreeBSD were artificially limited to PATH_MAX-2.

Add a short test case to verify the change.

Submitted by: Gaurav Gangalwar <ggangalwar AT isilon.com>
Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12589