fs-util: make chase_symlink() returns -ENOLINK when unsafe transitions are met

fs-util: make chase_symlink() returns -ENOLINK when unsafe transitions are met

We previously returned -EPERM but it can be returned for various other reasons
too.

Let's use -ENOLINK instead as this value shouldn't be used currently. This
allows users of CHASE_SAFE to detect without any ambiguities when unsafe
transitions are encountered by chase_symlinks().

All current users of CHASE_SAFE that explicitly reacted on -EPERM have been
converted to react on -ENOLINK.