[PATCH] uml: fix modify_ldt - missing break in switch

I am a lamer :-(. Luckily, Luo Xin performed LTP testing and found this failure.
Btw, the fact that the patch in which I introduced this was merged shows that:

a) I'm really trusted by people
b) sometimes they're wrong about point a).
c) lack of time for reviewers.

CC: Luo Xin <luothing@sina.com>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>