files-reg: restore PR_SET_DUMPABLE flag after setfsuid

Restore dumpable flag after setfsuid to assure that created /proc/self/*
file inode had task's credentials. Without it it would have root creds
and trying to access proc files of task will fail from non-root user
in generic vfs permission check.

Signed-off-by: Dmitry Safonov <dsafonov@odin.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>