fix uml-x86_64

__NR_syscall_max is done in x86_64 asm-offsets; do an equivalent in
uml kern_constants.h

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>