fsnotify: merge inotify wd image into inotify image (v4)

All watch descriptors are collected in a list and then
they are written in inotify image as a repeated field.

This images merge reduces the amount of image files criu
generates and may simplify the fix of mentioned above issue.

v2: use free_inotify_wd_entry() instead of xfree in dump_one_inotify()
v3: don't leak ie.wd_entry
v4: save the original order of watchers
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>