Rework fixup_aufs_vma_fd() for non-AUFS links

This patch reworks fixup_aufs_vma_fd() to let symbolic links in
/proc/<pid>/map_files that are not pointing to AUFS branch names follow
the non-AUFS applcation logic.

The use case that prompted this commit was an application mapping
/dev/zero as shared and writeable which shows up in map_files as:

lrw------- ... 7fc5c5a5f000-7fc5c5a60000 -> /dev/zero (deleted)

If the AUFS support code reads the link, it will have to strip off the
" (deleted)" string added by the kernel but core CRIU code already
does this.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>