fts: Don't return FTS_SLNONE if it's not a symlink (if race).

When following symlinks, fts returned FTS_SLNONE when fstatat(flag=0)
failed, but a subsequent fstatat(flag=AT_SYMLINK_NOFOLLOW) succeeded. This
incorrectly triggered if a filename existed to be read from the directory,
was deleted before the fstatat(flag=0) and created again after the
fstatat(flag=0).

Fix this by only returning FTS_SLNONE if the result from
fstatat(flag=AT_SYMLINK_NOFOLLOW) is actually a symlink. If it is not a
symlink, treat it as if fstatat(flag=0) succeeded.

PR: 196724
Reported and tested by: pho
MFC after: 1 week