lseek() and truncate() on sparc really need sign extension

ftruncate() doesn't - it's declared with size as unsigned long,
but truncate() and lseek() have that argument as signed long.
IOW, these two really need sign extension + branch to native
syscall; argument validation in sys_... does *not* suffice.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>