vfs_lookup(): Minor performance optimizations

Refactor the symlink and mountpoint traversal logic to avoid
repeatedly checking the vnode type; a symlink cannot be a mountpoint
and vice versa. Avoid repeatedly checking cn_flags for NOCROSSMOUNT
and simplify the check which determines whether the vnode is a
mountpoint.

Suggested by: mjg
Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D35054