Sanitize the symlink name buffer size.

Currently, KIO uses lstat to get the buffersize for readlink.
But in certain situations, it returns an inappropriate value.

For example, "/proc/self" or "/sys/bus/cpu/devices/*" returns
its size is 0, and then readlink fails with EINVAL (so the
link won't be shown in KDE applications). TMSU seems to return
the target's actual filesize instead of the link size,
i.e. the length of the target's filename.

This patch limits the initial buffer size to sane values and
expands the buffer when needed. The behavior is similar to
ls, so it should be compatible with all filesystems.

REVIEW: 129259
BUG: 369275