asm-generic: make copy_from_user() zero the destination properly

commit 2545e5da080b4839dd859e3b09343a884f6ab0e3 upstream.

... in all cases, including the failing access_ok()

Note that some architectures using asm-generic/uaccess.h have
__copy_from_user() not zeroing the tail on failure halfway
through.  This variant works either way.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[wt: s/might_fault/might_sleep]

Signed-off-by: Willy Tarreau <w@1wt.eu>