compat: use sys_sendfile64() implementation for sendfile syscall

<asm-generic/unistd.h> was set up to use sys_sendfile() for the 32-bit
compat API instead of sys_sendfile64(), but in fact the right thing to
do is to use sys_sendfile64() in all cases.  The 32-bit sendfile64() API
in glibc uses the sendfile64 syscall, so it has to be capable of doing
full 64-bit operations.  But the sys_sendfile() kernel implementation
has a MAX_NON_LFS test in it which explicitly limits the offset to 2^32.
So, we need to use the sys_sendfile64() implementation in the kernel
for this case.

Cc: <stable@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>