Handle __copy_from_user() return value.

Noticed by inspection, not from build warning.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Markus Lidel <Markus.Lidel@shadowconnect.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>