files: Rename variables in collect_fd

Variables `l' and `le' are short and named
too similar which makes them hard to distinguish.

Thus rename `l' to `le' (list entry) and `le' to
`new_le'. This makes them easy to recognize.

Also add braces around list_for_each_entry.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>